thanks for the replies all

Just to clarify and respond to some:
I am a sprinter so yes, I shouldn't necessarily be "good" at a distance race or ride. I live in Boulder and have been on the usual program of long rides up canyons since I moved here (age 37). Even though I've never been much of a performer (my bike-racer wife would have to wait for me at the top usually), I have noticed that the issues with me doing long stuff and climbing simply gets worse and worse every year - to the point of it being painful - literally and figuratively.

I have been in the gym more in the last few years in order to get better at track racing but I still like to do crits on occasion. Last crit I did in 2016, I couldn't finish - very frustrating. My muscles are getting bigger and stronger and this fact could be contributing, maybe. I get the sensation that when I go for a 1,500+ watt effort that my muscles comply but then say "ok, done".

As far as the doctor goes, we did around 30 blood tests and a pee test. All came back nominal for anything 'wrong'. I'm willing to bet that if I had a million $ to throw at him and find the cause, we'd turn up something in my muscles or Krebs cycle or ??? that points to the problem. Here's the dilemma right? - what value is it for a penta-genarian to get to the root of something that is obviously a unique and 'personal' problem in order for that old guy to be able to finish a crit? Not much truthfully, but still!

Thanks again for the input.