I'm 56, retired 12 yrs ago, and I can still max out the USMC PT test, and I was in the USAF. I'm not a fanatic about it either; walk the dogs every day, run/bike 3X week, some martial arts andcalisthenics 2X week.
Motivation and accountability is the key, and the best is internal, not external. Figure out how to push yourself instead of having somebody else do it for you. Easiest way to do that is find something(s) you like to do, then you really don't have to push whatever it is, it/they will pull you. I know that is easier said than done. Lucky for me, I like to do the simple stuff that's good for me. I don't "work out", I play. I smile when I sweat. I make time for things I am looking fwd to, don't look for/jump on excuses to skip something I am not.
Or ya could just say heck w it, eat, drink, and be merry until the pain in your arm and lights out. There's a good argument to be made that a shorter life that's more fun beats a longer life that isn't. I see too many people in the gym and on the road who do NOT look like they are having any fun at all! Whats the point if they are not? How much you smile and laugh might have more to do w how long you live than how much you grunt and groan anyway...