I'm 70. I've always had slow muscle recovery. Also easily fatigued. Had trouble making it through a 9-5 day, which fortunately I didn't have to do after age 33. Played tennis in my forties. Again, long recovery time compared to peers. Had heart surgery in 2015 with profound general fatigue afterwards. My heart is fine now. Since then, I need to lie down after being on my feet for 30 minutes or sitting for 2-3 hours. I do however progress with weight-lifting and short sprints. I get stronger and faster. Just no endurance. I don't have the cognitive problems that chronic fatigue syndrome folks have, but physically my symptoms, post-exertional malaise, are very similar.