A big complaint about exercising is that most people just don't have time. Well, you definitely have the time when you're doing time. Some real prisons might have actual gyms, especially white collar prisons. However, this trope usually involves the prisoners working out in their own cells, often in secret - especially if it's preparing for busting out. Characters who're already tough as nails might use the time to read a lot of books. So the lesson is, however you go in, fictional prison is a Badass Bookworm factory. Compare/Contrast Training Montage and Prison Riot.