Progressive 15 Day Workouts

At the end of the 15 days (and maybe at points before, e.g., at 5 and 10 days) ask the user how challenging they found the workout from not challenging, through moderately challenging, to very challenging. If the user scores it as not challenging, increase the difficulty (number of reps, difficulty of exercise, suggest increased weight, etc ). If the user scores are very challenging, do the opposite and make it less challenging. And if the score is moderately, then keep things the same.