IRONMAN 70.3 · Boulder · 12 Jun 2027
How to train for IRONMAN 70.3 Boulder
The organiser's own summary of this race reads swim, reservoir, bike, hilly, run, hilly. People take the word at face value and spend the spring doing hill repeats. Then they get to Colorado and find that the hills are not where the day goes.
Boulder is a heat race held at altitude on a course that climbs slowly and falls quickly. The steepest sustained rise on the bike costs 0.6 percent of the leg. The heat costs a great deal more, and it is waiting on the run.
By Kian Briel, founder. Analysed by engine 1, updated
What is actually on the road
The organiser's own course files, drawn the way the Event Analyzer draws a member's race.
- Swim1.97 km0 m upCourse file
- Bike90.5 km in 2 laps614 m upCourse file
- Run21.7 km in 2 laps120 m upCourse file
In the Boulder Reservoir you swim one counterclockwise loop, 2 km from the east boat ramp to the west boat ramp, in water the organiser logged at 22 C in 2026 and 20 C two years earlier.
The bike is 2 laps of the same loop. You leave the reservoir onto Diagonal Highway and head for Foothills Highway with the Flatirons in front of you, a long gentle rise broken by short rollers. The first 10.4 km average 0.9 percent, which nobody would call a hill and everybody feels. A right turn onto Hygiene Road starts the payback. From there to 63rd Street it is downhill and fast, with pitches of 6.5 and 5.4 percent and a 1.7 km stretch at 4.4 percent, and then the flat run home past the reservoir gates to start it all again. 614 m of climbing across the whole ride.
The run is 2 laps, counterclockwise around the reservoir. East toward the dam, north on the trail, a short piece of 63rd, then Monarch Road and 55th, both packed dirt, before you come back in through the north entrance past transition. 120 m of climbing in 21.7 km. The organiser calls it downhill rollers. The profile calls it flat: the two longest run segments, 44.5 and 37.6 percent of the leg, both read level.
Course shape by distance
- Flat 92%
- Climbing 3%
- Descending 6%
The longest stretches
- Bike, climbing0.22 km1% of the bike
- Run, flat9.64 km45% of the run
- Run, flat8.1 km38% of the run
Each row is one unbroken stretch of the course that keeps the same shape the whole way. The number is how much of that leg you spend inside it, and it comes out the same whether you are fast or slow.
What this course asks of each leg
Twelve training dimensions scored for each leg against its own course, the way the Event Analyzer scores a member's race.
First on the list is environmental resilience, scored 90 on every leg and ranked ahead of anything the terrain produces. The organiser's own record explains it: highs of 33 C, 31 C and 35 C across the last three editions, on a course the organiser sells as high elevation. Behind that sit the demands you would expect of a half-distance triathlon with a long bike: aerobic endurance at 85 and muscular endurance at 82 for the bike leg, with threshold capacity at 70 and the two durability scores in the mid sixties.
The rollers show up in one place. High-intensity capacity reads 54 on the bike against 40 on the run and the swim, and strength and force reads 42 on the bike. That is the short rises out on Diagonal Highway asking for something above steady power, briefly, many times. The engine turned the bike file into a session of nine efforts of three quarters of a minute, each followed by close to six minutes of easy riding, and that spacing tells you how short and how far apart those rises are.
Put the bands on it. A Trained athlete swims 39:31, rides 3:05:58 at 182 W and runs 2:02:53, a day of 5:58:22 with the bike at 52 percent of it. The Competitive band finishes in 5:13:06 from 4.15 W/kg at 75 kg and 4:20 /km threshold pace. Its bike is 2:44:31. Its run is 1:44:25. The run is the larger gain for the time it fills, and it is gained in the hottest part of the day.
- Environmental resilience90
- Aerobic endurance85
- Muscular endurance80
- Technical skill74
- Threshold capacity70
- Aerobic durability65
- Threshold durability65
- Fueling resilience55
- High-intensity capacity40
- Anaerobic capacity25
- Strength / force20
- Sprint power5
Nymiva reads what you can already show, from your own data
Connect your training history and the Focus Lab scores the same twelve dimensions this course demands against where you sit today. The gap between the two is what you train.
- MeasuredFrom your thresholds and testsThreshold power, threshold pace, critical swim speed and field tests you have done. The dimensions they can carry are scored directly.
- EstimatedFrom your training historyConnected activities are read for what they prove: long efforts, late-session fade, repeated hard work, fueling on long days. Each score says how it was read.
- ProxyFrom a stand-in, and named as oneWhere only an indirect signal exists, the score carries the proxy it came from and a lower confidence, never a number dressed as a measurement.
- UnmeasuredHonestly blankA dimension nothing in your data can answer stays unmeasured. It is never a zero, and the plan does not pretend to know it.
Every dimension ends in one word
- Develop the course asks more than you can show
- Maintain close enough to hold at the minimum dose
- Covered nothing to build here
- Unmeasured the course needs it, your data cannot say yet
Nymiva reads twelve dimensions off the course and the same twelve off the athlete. The course side comes from the file you see on this page. The athlete side comes from thresholds and connected training history, and each score carries a note on its source and how much weight it can bear. Only where the course asks for more than the athlete can show is there a gap, and a gap on the bike counts for more here than one in the water, because the bike fills half the day and the swim about a ninth of it.
This page knows nothing about you, so its season takes the plainest assumption: your gaps sit where the demands are largest. It builds 24 weeks from that, 5 developmental blocks and a two-week taper. Aerobic endurance opens the season in all three sports. Threshold durability leads the second block on the bike and the run, while the swim works on threshold capacity. The third block swaps them, with swim technique at the front. Muscular endurance leads the fourth, and aerobic endurance returns to lead the last block before the taper. A member's season starts from their own numbers and will not look like this one.
Train the bike to protect the run
Which demands matter most across the whole race. A leg's demand counts for as much of the day as that leg takes, so a score of 85 on the bike outweighs the same score on the swim.
- 01Environmental resilienceBike leg, 52% of the day90 × 52%47
- 02Aerobic enduranceBike leg, 52% of the day85 × 52%44
- 03Muscular enduranceBike leg, 52% of the day82 × 52%43
The bike takes 52% of the Trained reference band’s day, so its demands carry the most race weight. What the bike leaves in the legs is what the run gets.
Which demand gets a block, and when
The engine freezes one lead focus per sport on every developmental block, holds the rest at maintenance, and verifies before it reprioritises.
An illustration, not your plan. This page has none of your data, so the season below is the engine’s answer to one stated assumption: that your gaps line up with what IRONMAN 70.3 Boulder demands. Your own season is built from your history, your thresholds, the days you can train and the races you have entered, and it will differ from this in its priorities, its sessions and its load.
- 01Run muscular endurance, a practice block4 weeks
- 02Run aerobic endurance, a development block4 weeks
- 03Bike muscular endurance, a practice block4 weeks
- 04Bike aerobic endurance, a development block4 weeks
In the order the allocator places them, general first, most race-like last.
- Week 1PrepPrep
A lead-in week. It develops nothing by design.
- Weeks 2 to 5Foundation 1Closes with a recovery week
- PriorityBike aerobic endurance
- PriorityRun aerobic endurance
- PrioritySwim aerobic endurance
- SupportBike muscular endurance
- SupportRun muscular endurance
- SupportSwim muscular endurance
- SupportSwim technical skill
- SupportBike threshold capacity
- SupportRun threshold capacity
- SupportSwim threshold capacity
- SupportBike threshold durability
- SupportRun threshold durability
- Weeks 6 to 9Foundation 2Closes with a recovery week
- PrioritySwim threshold capacity
- PriorityBike threshold durability
- PriorityRun threshold durability
- SupportBike aerobic endurance
- SupportRun aerobic endurance
- SupportSwim aerobic endurance
- SupportBike muscular endurance
- SupportRun muscular endurance
- SupportSwim muscular endurance
- SupportSwim technical skill
- SupportBike threshold capacity
- SupportRun threshold capacity
- Weeks 10 to 13Foundation 3Closes with a recovery week
- PrioritySwim technical skill
- PriorityBike threshold capacity
- PriorityRun threshold capacity
- SupportBike aerobic endurance
- SupportRun aerobic endurance
- SupportSwim aerobic endurance
- SupportBike muscular endurance
- SupportRun muscular endurance
- SupportSwim muscular endurance
- SupportSwim threshold capacity
- SupportBike threshold durability
- SupportRun threshold durability
- Weeks 14 to 17Event shaping 1Closes with a recovery week
- PriorityBike muscular endurance
- PriorityRun muscular endurance
- PrioritySwim muscular endurance
- SupportBike aerobic endurance
- SupportRun aerobic endurance
- SupportSwim aerobic endurance
- SupportSwim technical skill
- SupportBike threshold capacity
- SupportRun threshold capacity
- SupportSwim threshold capacity
- SupportBike threshold durability
- SupportRun threshold durability
- Weeks 18 to 21Event shaping 2Closes with a recovery week
- PriorityBike aerobic endurance
- PriorityRun aerobic endurance
- PrioritySwim aerobic endurance
- SupportBike muscular endurance
- SupportRun muscular endurance
- SupportSwim muscular endurance
- SupportSwim technical skill
- SupportBike threshold capacity
- SupportRun threshold capacity
- SupportSwim threshold capacity
- SupportBike threshold durability
- SupportRun threshold durability
- Weeks 22 to 23PeakPeak
The taper. Load comes down toward the race, content stays specific.
- Week 24RaceRace
Race week.
Held at maintenance between blocks
- Runmuscular endurance, aerobic endurance, aerobic durability, threshold capacity, threshold durability
- Bikemuscular endurance, aerobic endurance, aerobic durability, threshold capacity, threshold durability
- Swimaerobic endurance, aerobic durability, threshold capacity, threshold durability, muscular endurance, technical skill
Touches scheduled by each capability’s own decay, at a fraction of its development dose, so earlier work is still there on race day.
The sessions the blocks are built from
One entry per kind of work the block trains, at the intensity that work is meant to be done at. Where the same work runs at several lengths, the entry shows the span it grows across rather than one entry per length.
- Foundation 1 · Weeks 2 to 575 to 240 min
150 to 210 min at Zone 2
BikePower, Zone 2Trains aerobic capacity - Foundation 1 · Weeks 2 to 540 to 270 min
180 to 210 min at Zone 3, then 30 min at Zone 4
BikePower, Zone 3Trains aerobic capacity - Foundation 1 · Weeks 2 to 535 to 120 min
90 min at Zone 2
RunPace, Zone 2Trains aerobic capacity - Foundation 1 · Weeks 2 to 550 min
4 x 1.5 min at Zone 3, 6 min easy between
RunHeart rate, Zone 3Trains aerobic capacity - Foundation 1 · Weeks 2 to 545 to 51 min
4 x 8 min at Zone 2, 1 min easy between
SwimPace, Zone 2Trains aerobic capacity - Foundation 1 · Weeks 2 to 547 to 81 min
3 x 1000 m at Zone 3, 0.3 min easy between
SwimPace, Zone 3Trains aerobic capacity
9× rolling-course surges
0.8 min at Zone 5, then 0.8 min at Zone 5, then 0.8 min at Zone 5, then 0.8 min at Zone 5, then 0.8 min at Zone 5, then 0.8 min at Zone 5, then 0.8 min at Zone 5, then 0.8 min at Zone 5, then 0.8 min at Zone 5, 5.8 min easy between
Compiled by the deterministic course compiler from the organiser’s file: the course’s one repeatable requirement, rehearsed at a dose the progression allows.
- Bike
- 78 min
- Trains threshold durability
- Supports muscular endurance
- Supports high intensity capacity
24 weeks, laid out backwards from race day
The shortest runway Nymiva builds for this event type, with its blocks, recovery weeks, taper and race week in place.
- 1
- 2
- 3
- 4
- 5
- 6
- 7
- 8
- 9
- 10
- 11
- 12
- 13
- 14
- 15
- 16
- 17
- 18
- 19
- 20
- 21
- 22
- 23
- 24
- Prep
- Foundation
- Event shaping
- Peak
- Race
- Recovery week
Your own season carries these same phases on your own dates, with the load solved from your fitness and the days you can train, and it changes when your data does.
What each level does on the day
Reference finish times per peer band, the pacing plan the model holds each band to, and where the day is decided.
| Band | Swim | Bike | Run | Finish |
|---|---|---|---|---|
| Recreational2:11 /100m critical swim speed · 2.47 W/kg at 75 kg · 6:08 /km threshold pace | 0:46:38 | 3:21:52 | 2:27:47 | 6:46:17 |
| Trained1:51 /100m critical swim speed · 3.03 W/kg at 75 kg · 5:06 /km threshold pace | 0:39:31 | 3:05:58 | 2:02:53 | 5:58:22 |
| Competitive1:36 /100m critical swim speed · 4.15 W/kg at 75 kg · 4:20 /km threshold pace | 0:34:10 | 2:44:31 | 1:44:25 | 5:13:06 |
| Elite1:26 /100m critical swim speed · 5.27 W/kg at 75 kg · 3:50 /km threshold pace | 0:30:37 | 2:30:15 | 1:32:22 | 4:43:14 |
Pacing plan
- 0 km0.9%26.5 km/h23:35
- 10.4 km-3.1%45.8 km/h0:13
- 10.6 km-2.8%44.4 km/h0:29
- 10.9 km4.5%13.9 km/h0:58
- 11.1 km0.2%29.8 km/h5:50
- 14 km3.9%15.4 km/h1:13
- 14.4 km0.4%28.8 km/h5:47
- 17.1 km3.6%16.2 km/h1:01
33 more segments in the full plan.
- Run: reach 4:20 /km and this course gives back 45:16.
Where it is decided
On the drag out toward the Foothills, twice.
Each lap climbs slowly from the reservoir almost all the way to Hygiene Road and then gives it back in a few minutes on the far side. Across the two laps, the way up takes about half of a Trained athlete's bike time. The descents take a small fraction of it. That imbalance is the whole bike course. Time spent climbing at 0.9 percent is time you can buy with steady power. Time spent descending at speed is time nobody can buy at all.
So the result is set by what you hold on a road that does not look like a climb. The short rollers invite you to push them. Hold 182 W through them and on the long rise between them, and freewheel the descent without apology.
Then comes the part the organiser's word hilly never mentioned. A Trained athlete starts the run close to four hours into the day, partly on dirt, around the reservoir, in the heat. The engine puts 44.5 percent of the run leg on one unbroken flat segment. Whatever the bike cost you, that is where you pay it.
Getting it wrong
Training hills instead of heat. The course is 91.6 percent flat by distance, the steepest climb is 4.5 percent, and strength and force sits far below aerobic endurance on every leg. Nobody arrives in Boulder undertrained for its gradients. Plenty arrive undertrained for a half marathon at 35 C.
Racing the first lap of the bike. The outbound drag feels easy with fresh legs and a full field. Riders hold a number they cannot repeat on lap two, and the second climb toward the Foothills is where they find out.
Believing the word downhill on the run. Monarch Road reads as rollers on the organiser's page and as level ground on the profile. Run the first lap at the pace the description suggests and the second lap on 55th will be long.
Conditions on the day
- 2026high 33 C, low 10 Cwater 22 C
- 2025high 31 C, low 14 Cwater 21 C
- 2024high 35 C, low 15 Cwater 20 C
From the organiser’s own records. Not a forecast.
Every session is a claim, every block is a test
A plan is not a calendar. Each key session states what it trains and how that is checked, and each block ends with a public verdict before the next one is chosen.
What a block promises
Bike aerobic endurance moves from your baseline to a stated target by the end of Foundation 1
- The baseline is your measured score when the block starts. The target is the score the model predicts for it.
- Verified at the block’s recovery week, by a field test or a refresh of the supply read from your history.
- Confidence rises by agreement between independent signals, never by one metric crossing a line.
- A block that misses its target is a public miss. The next block is chosen from what was proven, not from what was planned.
- 01Stimulus achievedThe work landed in the zone the session named, for the dose it named.
- 02Partially achievedSome of the work landed. The session banks what it can prove.
- 03Not achievedThe load may have been there. The stimulus was not, so nothing is banked.
- 04Data insufficientNo usable stream. The session is neither credited nor blamed.
- Session verified
- Progression banked
- Block verified
- Priority reassessed
- Plan adapted
Your numbers, this course
Analyze IRONMAN 70.3 Boulder against my fitness
The same simulation and the same Focus Lab read, on your thresholds instead of a band’s. Three numbers give a finish estimate and a partial read. A connected history gives the whole table.
The first 14 days are free.
- Trained band finish
- 5:58:22
- Of the day on the bike
- 52%
- Dimensions scored per leg
- 12
- Season runway shown
- 24 wk
Questions people ask
- How hilly is the IRONMAN 70.3 Boulder bike course?
- The organiser calls it hilly, and the file says rolling. Two laps of about 45 km climb 614 m in total, the steepest sustained rise is 223 m at 4.5 percent, and most of the climbing is a gentle drag out along Diagonal Highway toward Foothills Highway before a fast drop from Hygiene Road to 63rd Street.
- What is the water temperature in the Boulder Reservoir?
- The organiser logged 22 C in 2026, 21 C in 2025 and 20 C in 2024. The wetsuit decision is not on the organiser's course page, so check the athlete guide in race week.
- Is the IRONMAN 70.3 Boulder run on trails?
- Partly. The two-lap run leaves transition toward the Boulder Reservoir dam, follows the trail north, then runs Monarch Road and 55th, both hard packed dirt. It climbs 120 m over the whole half marathon.
- How hot does IRONMAN 70.3 Boulder get?
- The organiser recorded highs of 33 C in 2026, 31 C in 2025 and 35 C in 2024. The run is where that heat lands, because a Trained athlete starts it close to four hours into the day.
Course data from the organiser’s published files, analysed 24 Sept 2026 by engine 1, demand model 4. Reference times are simulated for a peer band at a stated reference athlete and are not a prediction for anyone. Nothing on this page is medical advice.
IRONMAN® and IRONMAN 70.3® are registered trademarks, and 5150 is a trademark, of World Triathlon Corporation. Nymiva is an independent training platform and is not affiliated with, sponsored by or endorsed by World Triathlon Corporation. Course data is read from the organiser’s public course pages for the purpose of describing the event.