There are two workouts in the Athlete Fit program, both free.
The requirement is to complete each of them twice, which means four times in total.
You can simply start the workout, wait till it starts the first exercise, then pause and skip to the end of the workout. That counts as one completion. Doing all four shouldn't take more than 10 mins.
The challenge grants you four loyalty stamps.
Per MathGuy24: As usual, the stamps are not awarded until the next workout. You can skip to the end for the 5th workout also, which can be any workout.