The primary distinguishing feature of running in a triathlon is that it occurs after the athlete has already been exercising in two other disciplines for an extended period of time, so many muscles are already tired.

The effect of switching from cycling to running can be very profound; first-time triathletes are often astonished at the bizarre sensation in their thighs a few hundred yards into the run and discover that they run at a much slower pace than they are accustomed to in training.

Triathletes train for this phenomenon through transition workouts or "bricks": back-to-back workouts involving two disciplines, most commonly cycling and running. (The term "brick" is derived from a partial anagram of Bike-Run.)

Source: Wikipedia