Over exertion will do it all by itself and is not always easy to recognize. So is a lack of sleep.
I ended up being hospitalized twice for over exertion and the last time was serious with a diagnosis of Rhabdomyolysis with kidney failure. . With an honest self evaluation I came to the conclusion that I get so self absorbed that I could not guarantee that it would not happen again. Bottom line was I needed a seriously strong shift in direction. Not what I wanted at all and was unwilling to give up on what I considered fun. My answer was an e bike that allowed me to could carry more stuff for good support and still keep my heart rate under stupid.
That change opened up new exciting horizons like Fat Bikes and Dirt Touring and more away from technical single track. I could not be happier and have not been hospitalized since.