I do Peloton classes a few times a week (I know) and do indoor rock climbing as a second hobby. Keeps me in good fitness and with decent enough bodyweight strength. Periodically do yoga to stay semi-flexible (though that could always be better). Main thing I think about is fighting against going stiff and/or limp and having a level of fitness well enough to make it walking through a whole 18 without crashing and burning at the end (any more than is normal after a few hours of sport). But at the end of the day, anything is better than nothing! A walk around the neighborhood, a few sessions at the gym per week -- whatever makes sense for you and your fitness level at the time of reading.