Edit: (Credit: FNSUITE GHOST) Unlike the first Escapists game, your stats don't decrease over time.
This can be done in any prison in the gym area.
(Yoiu will have excersise time in your schedule so you don't have to grind the achievement)
Choose the "Workout: Fitness" to start the mini-game.
To get energy back fast you can consume food items, lay on a bed, get a shower or sit in a chair.
Once you have 100 Fitness the achievement will pop.