I can look back on a long time of working (at various motivation levels, to be sure) of working out and training, and I am really glad I did. Although the primary reason for maintaining and increasing fitness was to be able to enjoy the outdoors, the benefits came in many other parts of my life, as well.
Read just about anything on maintaining mental ability, kidney function, whatever, and the prescriptions are very similar and contain these elements - eat reasonably (less red meat and plenty of veggies), no smoking, very moderate alcohol consumption, (if any) and exercise!
About fifty years ago, I started paying increasing attention to fitness, especially running, and looking back, I am glad I did - not so much because I was still able to get about easily in the outdoors, but because of the more general and significant benefits, as well - mostly avoidance of a lot of the degenerative conditions that accompany old age (not all, but most).
I don't know how distant you are from a decent trail head, but I hope there is something available which will allow you to get out alone in reasonable safety. Also that you have some other reasonable options for maintaining fitness.