First of all, I'm no doctor. If you feel as if you're body isn't reacting properly to the stresses you are putting yourself through ask your doctor about it. Maybe some detailed blood work is warranted. It could be something as simple as a vitamin deficiency.
I think we can all agree that we're not the person we were 30 years ago. But, I think following a regiment of consuming sensible healthy foods, staying hydrated, stretching and exercising within our abilities is about all you can do. I like to alternate my bike days with gym days. Sometimes I alternate my gym days with a walk in the park day. Any movement is good.
But don't beat yourself up about the need for a nap, we've all been there. some of my best naps have been after a unusually strenuous ride.