How to pace your first Ironman
Last updated June 2026 · ~7 min read
Almost every first-timer makes the same mistake: they go out too hard and pay for it on the run. A long-course triathlon isn't won in the first hour — it's survived there. Here's how to pace the swim, bike and run by effort, not ego, so you cross the line strong.
1. The swim — relax and stay aerobic
The swim is only ~10% of your day, so there's nothing to win here — but plenty to lose. Sprinting off the start spikes your heart rate and can leave you gassed for hours.
- Start steady. Settle into a rhythm you could hold "all day." Breathing should feel controlled within the first few hundred metres.
- Sight and draft. Swimming on someone's feet saves real energy. Sight every 6–8 strokes so you don't add distance weaving off course.
- Know your number. Use the swim pace calculator to turn your pool pace per 100 m into a realistic swim split — a 2:00/100 m swimmer covers the full 3.8 km in around 1:16.
2. The bike — discipline wins the race
The bike is 50%+ of your time and where the day is decided. The goal is to arrive at T2 with legs left to run.
- Ride by power or heart rate, not feel. Target roughly 65–75% of FTP (Intensity Factor 0.65–0.70) for a full, or 75–85% for a 70.3. By heart rate, that's low Zone 2 to Zone 3.
- Cap the early surges. Don't chase riders who pass you in the first 30 miles — your race is your own. Hold your number especially on climbs.
- Fuel from the start. Begin eating and drinking in the first 15 minutes; the bike is where you bank the calories the run will burn.
- Project your split. Plug your realistic average speed into the bike split calculator to see your bike time and what it means for the cutoffs.
3. The run — start slow, then hold
A well-paced marathon off the bike feels almost easy for the first few miles — that's correct. It should feel hard by the end no matter what.
- Hold back early. Run the first 3–5 miles slightly slower than goal pace. Adrenaline will make easy feel too slow — trust it.
- Walk the aid stations. A deliberate 10–20 second walk through each aid station to actually drink costs little and prevents the late-race blow-up.
- Negative-split if you can. Finishing faster than you started is the signature of good pacing. Fading hard means you overbiked or under-fueled.
Build your race-day splits
Pacing is a plan, not a vibe. Set a realistic goal, then work out the swim, bike and run splits that add up to it — and check them against the cutoffs:
Frequently asked questions
What's the most common Ironman pacing mistake?
Overbiking. The bike is where the race is won or lost — ride 5–10% easier than feels right and you'll keep your legs for the marathon. Push too hard and the run unravels.
What power or heart rate should I ride at?
Around 65–75% of FTP (IF 0.65–0.70) for a full or 75–85% for a 70.3, holding low Zone 2 to Zone 3 heart rate. How the run feels tells you whether you nailed it.
How should I pace the run?
Start slower than goal pace for the first 3–5 miles, walk aid stations to drink, then hold steady or build. A negative split means you paced the whole day well.