Running is the last event in an Ironman

In 1974, when the San Diego Track Club held the first one in modern history, triathletes would run first, than bike, and then swam. Running came first because it was what came naturally to the athletes at the time; thats all they did back then, run around. Back in Greek times, the term triathlon just meant a collection of three athletic events; they didnt have to be the three that we use now.

Run event switched from first to last for Safety Reasons

Because of safety reasons, modern triathlons run the events in the opposite order. You swim first, then you bike, and finally, you run that marathon. The idea behind this is that if runners fall down from exhaustion during a run, they fall to the ground. If you drop from exhaustion while youre in the ocean, chances are youre going to drown. Not a good way to attract contestants. So now, you run last.

Usually triathlons will have cutoff times throughout the race. For an Ironman, contestants must finish the swim component within 2 hours and 20 minutes and the bicycle component within 10 hours and 30 minutes of starting the race. That leaves six and a half hours for the run since contestants must finish the entire event in 17 hours. This gives strong runners a chance to distinguish themselves from the rest of the pack.