Gyms in the vicinity of whatever military base they were dumped outside of when they got out. That and martial arts classes.

Definitely met a shredded old guy or two. They exist, but the effort to maintain that type of physique is very time consuming at that age. The one I knew basically lived in the gym and didn't have much else going on.

But don't underestimate average looking old guys either, they're hiding decades of muscle build up under the wrinkles. If they lived a healthy active life, made it to 70 without a major surgery, heart condition or something like chemo, they're likely physically stronger than you'd assume.

This image is at best AI generated, at least CGI