The 70.3 is won — and lost — in the back half of the run. Almost every blown race I've coached or raced traces back to one mistake: the bike cost more than the runner could afford. This guide is the framework I use with my 70.3 athletes to set Intensity Factor on the bike, translate VDOT into a realistic half-marathon split, and fuel both legs without blowing up.
Before you read further, plug your recent open race into the 70.3 time predictor — the targets below assume a realistic goal time, not an aspirational one.
Swim — 1.2 mi at threshold minus one
The swim is the smallest cost in your 70.3 day. Pace it at roughly your threshold pace per 100 minus 3–5 seconds — fast enough to draft a strong group, slow enough that heart rate is fully recovered by the time you've zipped your helmet. If you exit the water with a max HR, you've already spent bike watts you'd rather have at mile 50.
Bike — Intensity Factor and TSS targets
Intensity Factor (IF) is your normalized power divided by your FTP. Training Stress Score (TSS) is the cost of the ride. For a 70.3, both numbers matter — IF tells you whether the effort is sustainable; TSS tells you whether the run will survive it.
| Athlete profile | Target IF | Target TSS | Coaching note |
|---|---|---|---|
| First-timer / limited run base | 0.72–0.78 | 165–190 | Protect the run. The bike is a delivery vehicle. |
| Mid-pack age-grouper | 0.78–0.82 | 185–210 | The standard sustainable window. Best ROI for most. |
| Strong runner, deep base | 0.80–0.83 | 195–215 | You can spend more on the bike because your run won't collapse. |
| Cycling-dominant, podium hunt | 0.83–0.85 | 205–225 | Only if you've rehearsed 5+ long bricks at this IF. |
- Variability Index (VI) under 1.05. Surges on rollers and into headwinds cost more than the average power suggests. A smooth ride at 0.80 IF beats a spiky ride at 0.78 IF every time.
- Cap power on climbs at IF + 0.10. If you're targeting 0.80 IF, climbing watts should not exceed 0.90 of FTP for any sustained section. The matches you burn here come out of mile 9 of the run.
- Last 20 minutes of the bike: drop to IF − 0.05. Get HR down 5–8 bpm before T2 so you can run rather than jog the first mile.
Run — VDOT-based pacing for the 13.1
A 70.3 run is not an open half-marathon. The right reference pace is your open marathon (M) pace from VDOT, not your open half pace. Off a well-paced bike, most athletes run their 70.3 within 0–60 seconds per mile of their open M pace.
| Phase | Distance | Target pace | Cue |
|---|---|---|---|
| 0–5K (settle) | 0–3.1 mi | M + 15 sec/mi | Legs feel terrible. They will. Hold back deliberately. |
| 5K–10K (find rhythm) | 3.1–6.2 mi | M + 10 sec/mi | Cadence opens up. HR should drift up by 3–4 bpm only. |
| 10K–15K (commit) | 6.2–9.3 mi | M + 5 sec/mi | If fueling is on, lock in. If not, hold the previous pace. |
| 15K–finish (deliver) | 9.3–13.1 mi | M pace or faster | Drop 2–3 sec/mi every aid station. Empty the tank in the final mile. |
Don't know your current VDOT? Drop a recent race into the VDOT guide to set your zones, then return here for the 70.3-specific targets.
Fueling — what holds the pacing together
- Bike: 90–110 g carbs/hour. Combine a high-carb drink (60–80 g/hr) with a gel or chew every 25–30 minutes. Trained, modern guts handle 110 g/hr; untrained guts crater.
- Bike: 600–900 mg sodium/hour in hot/humid conditions. Use a single concentrated bottle rather than course nutrition you've never tested.
- Run: 60–80 g carbs/hour. A gel every 20–25 minutes plus aid-station Coke from mile 8 onward is the simplest reliable protocol.
- Pre-race: 1.5–2 g/kg carbs 3 hours before, then 30–40 g 15 minutes before the swim start. This is the most overlooked lever in the back-of-pack-to-mid-pack jump.
The two-rehearsal rule
Don't race the targets above on race day if you haven't rehearsed them in training. Run two key bricks in the last 6 weeks: a 56-mile bike at goal IF + 5K run at goal pace, and a 3-hour bike at goal IF + 75-minute run at goal pace. If you can finish those rehearsals strong, the race math holds. If you can't, dial the bike IF down 0.02 — every time.
FAQ
What Intensity Factor should I target for a 70.3 bike?+
For most age-group athletes, IF 0.78–0.82 is the sustainable window that protects the half-marathon. Strong cyclists with a deep run base can push 0.83–0.85; first-timers and athletes off limited run volume should sit at 0.72–0.78.
How much TSS should the 70.3 bike leg cost?+
Plan for roughly 180–215 TSS on the bike. Above 220 TSS the run almost always falls apart in the back half — that's the difference between a 1:30 open half and a 1:50 off-the-bike half.
How do I set my 70.3 run pace from VDOT?+
Take your current VDOT, look up your marathon (M) pace, then run the first 5K of the 70.3 at M+10 to M+15 seconds per mile. If you can hold M+10 through 10K, you can usually drop to M pace for the final 5K.
How many carbs per hour on the 70.3 bike?+
90–110 g/hr is the modern target if your gut is trained for it. Below 80 g/hr you risk a glycogen-dump run; above 120 g/hr most athletes get GI distress without months of dedicated fueling practice.
Should I use heart rate or power on the bike?+
Power leads, heart rate confirms. Set your target by IF off FTP, then watch HR for drift — if HR climbs 10+ bpm at the same power on a flat course, you're either dehydrated, under-fueled, or about to blow up. Back off.
How is 70.3 pacing different from open-half pacing?+
An open half is paced at your half-marathon (HM) VDOT pace. A 70.3 run is paced 30–60 seconds slower per mile because you've already spent 180–215 TSS on the bike. Treating the 70.3 run like an open race is the most common reason athletes blow up at mile 9.
Want these targets dialed in for your race?
1:1 coaching means your IF, TSS, and run pacing are set off your current FTP and VDOT — and adjusted every week as you build into race day.