As you gain fitness, change your workout to keep it challenging and the results coming. Brian MacKenzie, a CrossFit coach who uses hill sprints with clients, recommends this progression:
1. Increase the number of reps by one per workout.
2. Gradually lengthen your sprints. Don't exceed 30 seconds per sprint.
3. Reduce the rest time between reps, but don't cut your recovery time so short your speed falls off.
4. Go to a steeper hill.
5. Mix up the format at random, as Agassi did, by sprinting backward or sideways, or breaking hills into segments of different lengths to keep your body guessing. Whereas Agassi hit the hill every day, once a week is enough for the rest of us.