Good that you say you are healthy, that's a good place to start from. I'm 51, and I personally follow the leangains program. I go to the gym 3 days a week and eat plenty (but differently than I did), so I think it's a really efficient way to gain muscle while also losing weight. I fell off the wagon a bit the 1st year of covid (gyms being closed sucked!), but I'm not back to where I was and then some. Check out r/leangains and/or read Martin's book for more info.
Couldn't pass up an old 1980's muscle bike reference, even for a rice cooker. :)
Anyhow, a critical metric of your fitness, especially over 50 crowed, is VO2 max. Read the article and do the Cooper test. It's only 12 min run on a track and see how far you get.
Here's an alternative calculator with several alternative tests
I did the 1.5 mile challenge and finished in 19 min 30 second. This gave me an abysmal 28 ml/kg/min , or "poor" rating. It wasn't long ago I could easily maintain a 10 min mile for 3 miles; or a 36 ml/kg/min VO2 max for a "good" rating at least. I've got some work to do.
There's plenty of advice in the article on how to improve V02 max. I originally did it by going out for a 3 mile walk and jog. After fast walking a bit to warm up, I'd try jogging until winded (say 30 seconds?). Went back to walking until I felt better. Jog again. Get pissed off at my poor perforce while walking. Repeat. Do this for the entire 3 mile distance, at least twice a week. Eventually, I was completing the 3 miles in 30 min and bored out of my skull, but at least satisfied with the accomplishment.
Misery loves company, so show your support by posting your scores - good or bad.