The split that decides the race is the second one
In a long distance triathlon the swim distributes few minutes and the run distributes many, but the one that decides the race is the bike. Riding 5 % above your sustainable intensity does not cost you five minutes on the bike: it costs you twenty or thirty in the marathon that follows, and in the worst case it costs you the whole race.
The reference used for this is intensity factor, the ratio between the normalised power of the bike leg and your threshold power. The ranges that work best in practice are these:
| Distance | Bike intensity (% of threshold) | Bike duration |
|---|---|---|
| Sprint | 92 – 98 % | 30 – 45 min |
| Olympic | 85 – 92 % | 60 – 80 min |
| Half Ironman 70.3 | 76 – 84 % | 2 h 20 – 3 h 30 |
| Ironman 140.6 | 65 – 75 % | 4 h 30 – 7 h |
Notice how little you ride in an Ironman: below three quarters of threshold. Almost everyone who blows up in an Ironman marathon does so because they rode ten points above that figure and felt wonderful for four hours.
Running off the bike costs, and it can be quantified
Nobody runs a triathlon at the same pace as a standalone race over the same distance. The penalty depends on three things: the distance (it grows with it), the intensity you rode at, and your accumulated experience of transitions. In an Olympic distance race you typically lose between 10 and 25 seconds per kilometre against your standalone 10 km; in an Ironman, the gap against your standalone marathon can exceed 60 seconds per kilometre.
Transitions are time too
In an age group triathlon T1 and T2 usually add up to between three and six minutes, and in an Ironman with bags and a change tent, easily eight or ten. That is time that shows up in the official result and that almost nobody includes when calculating a target, which produces that very common feeling of "I ran my planned pace and finished five minutes over".