70.3 Half Ironman Pacing: Breaking 4.5 Hours into 11 Decision Points

The Core Contradiction of 70.3

For age-group athletes, a 70.3 (1.9km swim, 90km bike, 21.1km run) takes 4:15–6:00. The challenge is that every 5% extra effort on the bike can cost you 4+ minutes on the run. The philosophy is “push the bike as hard as possible without sacrificing the run.”

Bike Power Ceiling

A VI above 1.06 indicates a surging riding style, which burns 8%–12% more glycogen than steady pacing.

11 Decision Points

1. Find your rhythm in the first 800m of the swim (1900 TT +4"/100)
2. Decide on socks in T1 beforehand
3. Ride 0–20km at 5W below target NP to settle your heart rate
4. At 20km: first gel + 250ml sports drink
5. At 40km: solid food
6. At 60km: second gel + electrolytes
7. At 75km: last big intake
8. At 85km: drop power IF −0.03 to wake up the running muscles
9. T2: pour water over your head to cool down
10. Run 0–3km at 8"/km slower
11. After 15km, treat each aid station as one checkpoint

Negative-Split Run

Ideally, the second half is 30–60 seconds faster than the first. A 1:35 runner covers the first 10.5km in 48:30 and the second half in under 47:00.

Hard Numbers for Fueling Flow

The 2:15–2:45 bike leg of a 70.3 is the golden absorption window: target 60–90g carbs, 500–750ml fluid, and 400–700mg sodium per hour, taken in small doses every 15 minutes—this reduces GI issues by 30%–40% compared to gulping large amounts every 30 minutes. On the run, gastric emptying worsens, so drop carbs to 40–60g per hour, switch to a gel + cola mix, and raise sodium to 600–900mg per hour to fight cramping.

Root-Cause Diagnosis of Three Types of Run Collapse

90% of “bike PB but total collapse” cases are the first type: an IF just 0.03 higher on the bike means glycogen runs out 25 minutes early.

I’ve seen too many people set a bike PB and then completely fall apart. The 70.3 mantra: it’s not about how fast you ride the bike, but how much you have left after you get off it. Treat the bike as saving money for the run, and you’re already halfway to winning.

Related Reading

• 70.3 Half-Ironman Finish Guide: Breakthrough Rhythm for Middle-Distance Athletes
• 70.3 Half-Ironman Race Strategy: Full Breakdown of Pacing, Fueling, and Mental Milestones
• Olympic-Distance to 70.3 Transition Training: The Advanced Path into Half-Ironman
• IRONMAN 70.3 Half-Ironman Training Plan: 24-Week Periodized Race Preparation