Most age-group 70.3 races are lost in the first 30 minutes of the bike. The athlete comes out of T1 with a fast swim split, a heart rate still elevated from the water, and 89 other riders in their eyeline. They push the first climb. They hold the wheel. Twenty minutes in, Normalised Power is sitting at 82% of FTP instead of 74%, and the damage is already done — they just won't feel it for another three hours.
The half Ironman bike is not a time trial. It is a pacing exercise with a 21.1km run attached. The bike leg exists to deliver you to T2 with enough glycogen, enough neuromuscular freshness, and enough gut function to run your actual half marathon pace.
This guide gives you the numbers: a wattage cap from FTP, a heart-rate ceiling as a backup, a fuelling timeline by 30-minute block, and how to adjust when the course or the weather goes sideways.
Why most 70.3 bike legs are over-cooked in the first hour
Post-race power files tell the same story across thousands of age-group athletes. The first-hour Normalised Power is 5–12% higher than the second-hour NP. That pattern almost always produces a positive-split run where pace drops 20–40 seconds per kilometre in the back half.
Three things conspire in that first hour. Adrenaline inflates heart rate by 5–10 bpm, so riders who pace by feel think they're cruising when they're actually over threshold. Packs form and break on early climbs, and the temptation to stay with a group pulls watts up. And the legs feel fresh because the swim is a low-power effort for the quads, so the first 20 minutes on the bike feels like a warm-up, not a race.
Dan Lorang, who coached Jan Frodeno and Gustav Iden through multiple world titles, has been explicit on the Roadman Cycling Podcast. The bike in a long-course race is about energy conservation, not output maximisation. His top athletes routinely ride the first 15 minutes 5–10% under target, not at it.
This matters because glycogen is finite. Every watt above your aerobic threshold in the first hour draws disproportionately from carbohydrate stores. You cannot replace that fuel at the rate you burn it. Overbike the opening, and the run is decided before you've hit the halfway turnaround on the bike.
Your wattage cap, and where it comes from
Start with a current FTP. "Current" means tested inside the last 6–8 weeks, not a number from last October. A 20-minute test or a ramp test both work; repeat the same protocol so the trend is honest. If you don't know your zones, use our FTP zones tool to convert an FTP into training and racing bands.
For a half Ironman, target 70–78% of FTP as Normalised Power across the full 90km. That range exists because athletes are not identical:
- 70–72% of FTP: first-time 70.3 athlete, bike-limited, or coming off a limited build. Intensity Factor around 0.72.
- 73–76% of FTP: the majority of trained age-groupers. IF 0.73–0.76. This is where most PRs are set.
- 77–78% of FTP: experienced athletes with high bike volume, strong run durability, and multiple 70.3 finishes behind them. IF 0.77–0.78.
Above 0.80 IF is professional territory and only works when the athlete has the run economy and fuelling capacity to back it up. For an age-grouper, 0.80+ is almost always a bike PB and a run blow-up.
Variability Index, NP divided by average power, should stay under 1.05 on flat courses. A VI of 1.10 means you're surging and coasting, which burns more glycogen than a steady effort at the same NP. Smooth power wins 70.3s.
If you want this built properly into a block that targets your A-race, triathlon bike coaching inside the Not Done Yet programme sets the cap based on your recent testing, your run durability, and the specific course profile.
Heart-rate as the sanity check
Power leads. Heart rate is the backup channel that catches what power can't see.
Set a heart-rate ceiling 5–8 beats below your threshold HR. For an athlete with a threshold HR of 170, that's a ceiling of 162–165. In the first 20 minutes, expect HR to run hot, 3–5 bpm above steady-state, because of swim exit, transition effort, and adrenaline. Don't chase it down with a power drop unless it's still elevated at the 30-minute mark.
What heart rate catches that power misses:
- Cardiac drift from heat or dehydration: HR climbs 8–10 bpm at the same power over 60–90 minutes. That's a fuelling and hydration signal, not a fitness signal.
- Fuelling failure: glycogen depletion pushes HR up as the body works harder to produce the same output from fat oxidation.
- Under-recovery: if your HR is 10+ bpm above expected at target watts from kilometre one, you arrived at the start line already depleted. Drop the wattage cap by 3–5% and salvage the run.
Prof. Stephen Seiler's work on internal versus external load applies directly here. Power is external load — what you produced. Heart rate is internal load — what it cost you. A mismatch between the two is always information worth acting on.
One caveat: heart rate alone is a bad pacer in the first hour. Racing nerves inflate it in ways that have nothing to do with effort. If you don't have a power meter, pace by perceived effort (conversational, just above endurance) for the first 20 minutes and settle in from there.
Fuelling timeline: 0–30 / 30–60 / 60–90 minutes
The bike is where you fuel the run. Total target: 70–90g of carbohydrate per hour, starting inside the first 15 minutes of the bike leg. Use our fuelling calculator to dial the grams-per-hour number to your body weight and expected bike time.
0–30 minutes. Start drinking by minute 10. Take your first gel or 25g carb dose at minute 15. HR will still be elevated from the swim, so don't force solid food. A carb drink at 60–80g/litre is the easiest way to get fuel in while the gut settles. Target: 30–40g of carbs in the bag by the 30-minute mark.
30–60 minutes. This is the main fuelling block. Gut is settled, power is steady, and absorption is at its best. Take a gel every 20–25 minutes alongside your carb drink. Sip every 5–10 minutes, not in big gulps. Target: another 70–90g of carbs across this block. By the 60-minute mark you should have 100–130g on board.
60–90+ minutes. Keep the rhythm. Do not back off fuelling because the legs feel good. That feeling is exactly what the fuelling produced. In the final 20 minutes of the bike, switch to liquid-only or a smaller gel to avoid arriving in T2 with a full stomach. Target: 60–80g more in this block.
Tim Spector's gut research points to a truth every long-course athlete needs to hear: fuelling tolerance is trainable. If 90g/hour wrecks your stomach today, train it. Practise race fuelling in every long ride for 8–12 weeks before the race.
Course-specific pacing adjustments
The 70–78% NP rule assumes a roughly flat to gently rolling course. Real 70.3 courses rarely cooperate.
Flat and fast (Barcelona, Des Moines, Western Sydney). Hold NP tight to your cap. VI under 1.04. The main trap is drafting packs. If you find yourself pulled above cap to stay with a group, let them go. A 90-second bike loss to a pack you couldn't run off is not a loss.
Rolling (70.3 Weymouth, Staffordshire). Allow climbs to spike 10–15% above NP cap, recover actively on descents. VI up to 1.07 is fine. Don't hammer the top of every roller. Crest the climb and roll over, don't sprint out of the saddle.
Hilly (Nice, Mont-Tremblant, St. George). NP cap drops 2–3% because the terrain demands more variability. Climbs can go to 95–100% of FTP on the steeper pitches but never above. VI up to 1.10. Descents are for recovering and fuelling, not for making time. Most age-groupers lose more time on the next climb than they gain bombing the descent.
Hot races (Kona 70.3, Cebu, Dubai). Drop the NP cap by 3–5%. Add 200–300mg sodium per hour on top of baseline. Expect HR to drift higher and accept it. Don't chase the HR ceiling by pushing more watts.
Dan Bigham's aero work is worth a note here: on a flat course, holding an aggressive TT position saves more time than any extra 10 watts would. Pace conservatively and stay aero. That's the combination that produces the fastest bike split you can run off.
What to do if the race doesn't go to plan
Races go wrong. Flat tyres, dropped bottles, stomach shutdowns, packs that blow up the plan. The question is whether the rest of your race goes wrong with it.
If you've gone out too hard in the first 30 minutes. Back the NP cap off by 5% for the next hour, not the next 10 minutes. You cannot un-burn glycogen, but you can limit further damage. Accept a slower bike split and protect the run.
If you've dropped a bottle or gel. At the next aid station, take two of whatever they're handing out. Don't try to make up calories in one block. Spread the catch-up across 30–40 minutes.
If your stomach shuts down. Stop solid fuel. Switch to Coke or a dilute carb drink only. Sip every 5 minutes. Getting 30g/hour in is better than 0g/hour because you tried to force a gel.
If your power meter drops out. Pace by heart rate at your ceiling minus 3–5 bpm, and by perceived effort: conversational with effort. Most experienced athletes pace within 2% of target by feel if they've trained it.
If it's clearly not your day. Finish the bike at endurance pace (60–65% of FTP) and either run for training or pull out cleanly. Blowing yourself apart to chase a time that's gone is how injuries happen.
Next step: pull your last three long-ride power files. Look at the first-hour NP versus the second-hour NP. If the gap is over 5%, your race-day pacing has a problem the legs can't fix — the plan has to.
