For exercise ever considered walking Up Hills, a quick way to get trim. Rock climbing or bouldering, canoeing, yoga, tai chi...dig holes in the garden ( not the botanical one!) For oldies, Cross training or just lifting weights is supposed to stop us withering away. There are so many ways to keep fit, if the mind is so inclined, anything to avoid running which is not good for old knees.
Discussion
Our holiday home, where I walk the most is very hilly, since being there, my stamina for hills has increased dramatically.
We also have a gym over the road that I helped get started, run by a friend, who constantly tries to get me to join.
I think walking is my go to exercise, I enjoy it and do it for pleasure, which is half the battle.