There’s a difference between the time when you realize that you’re no longer young and no amount of training or exercise will recover youth and the time when you see that the path before you is shorter and your end is over the horizon.
You’re still alpha in your fifties. You may know that youth is behind you and unrecoverable, but you’re capable and viril and seasoned by the lessons of the first half of your life. In my mid-fifties, I was an active skydiver, not a regular cyclist, yet I rode a bike 354 miles across Iowa in 7 days.
Revel in your fifties. Do those things that you can with verve. Perhaps as soon as the end of your sixties, mortality will come knocking. You’ll grumble that these things shouldn’t trouble you for another decade, but you’ll deal with them all the same.