jcsimco, at nearly the same age as you I encountered the exact same thing. Extreme fatigue after a ride, then within a year it became extreme fatigue during the ride and got to the point where 20 miles was more than I could do comfortably. The doctor reminded me that I am getting older and this can be expected. Not always the norm, but not unusual.
My health is fine, albeit I have slightly high blood pressure and control it will a very low dose of something I forget the name of. Been the same script for 15 years. Cycling keeps it in check for the most part.
Oddly enough at age 60 I figured out the problem. I am getting older. Been either racing, commuting, or touring since I was 15 years old so me and the doc know my conditioning is just fine. The issue is simply intensity. I can't ride like a 20 something anymore, and I should not be attempting to. My body knows what it needs and that is LSD. Long Slow Distance, thus that is what I do. I go any distance I want now without trouble as long as I ride at a pace my old bones are happy with. When pretend to be boy racer, I'm screwed.
Take it easy and enjoy the ride and the health benefits that go along with LSD.