61, can't ride a bike anymore without hurting myself for days.

It's not what I can't do, it's what I can do. Perhaps you need to pick up a new hobby. Perhaps you need to try an old hobby and reinvigorate it.

The hard part is giving up the runner's High. That's the addiction, and the loss you are feeling. I felt that very deeply when I had to give up skiing downhill, and quickly afterwards all snow activities.

I also felt that powerful adrenaline High tempered with steely calmness when I used to climb free solo, but I haven't even been on a rope in 20 years. Life changes. Adapt. Good luck.