Doesn't a triathlon involve running a marathon and other insane distances?

You are thinking of an Ironman triathlon, which consists of a 2.4-mile swim, 112-mile bike and a 26.2-mile marathon run.

I can assure you that Ironman triathlons are a small percentage of triathlons. The majority of triathlons are the “sprint-distance” variety, which include a ½-mile swim, 12-mile bike and 3-mile run.

There are also much smaller “Tri For Fun” type races.

The Olympic-distance is double a sprint-distance triathlon. The next step up is a half-Ironman.