Having the big 50 fade from my rear view mirror I understand what you are going thru.
My wife and I are both on the wrong side of mid-50s. Our observations is that we have to work much harder at the gym to stay even. Our bodies are not as resilient as when we were younger. The operative word is resiliency - our bodies' ability to absorb punishment and the ability to bounce back quickly. We find that we have to push our bodies much harder in training to keep the fitness and resiliency. Activities that requires both random movements and some sort of impact absorption (minor) seems to keep our bodies more resilient. Tennis, basketball and trail running comes to mind. It seems the gym exercise builds muscles but not the resiliency. The gym exercises are too predictable. The random movement & impact sports prevent our bodies from freezing up and keep loss of bone mass at bay. Keep the body guessing - stay one step ahead.
BTW, Osteoporosis also occur in men but not as drastic as in women.
As we grow older we tend to become more efficient at whatever we do. I can now ski the steep bumps of Taos without breathing heavy - can't do that in my 30's. But in my 30's I didn't know any better. It's as much a mind game as it is physical. Change things up - Do something different everyday. Learn something new everyday. Our coming January road trip from CO to NM is one of those different thing we are doing. We are only as old as we think we are. Be unpredictable. Look at the young folk and how many mistakes they make - most of them survive to make more. We have to make as many screw up as they do or we are not pushing it. If we stop pushing we grow old faster.
So just go out there and change things up a bit, a positive attitude goes a long way - it will come together.
All the above are my gut feels and observations. There is no scientific backing to any of it, but then I live and deal with my gut everyday. The doctors rather prescribe a pill for whatever and don't forget to schedule your next monthly with the receptionist on the way out.
My wife and I share a motto from the spoof movie Galaxy Quest - 'Never give up, never surrender'.
Just remember we are the 'Hell no we won't go quietly into the night' generation. Keep pushing.