Exercise HARD. Try doing some Zone 2 cardio for +1 hours, and see if this makes a difference. Or try some HIIT workout (Zone 5) and see how your body responds. A good overall workout machine is a rower. Try to set some kind of PR, like a 2k time trial. Push beyond what you think is possible and see how your body responds. My best sleeps are when I am physically exhausted - so weak that I could fall asleep on a couch (which is normally impossible for me).
I have some mixed feelings about this. I used to do this ~10 years ago when I was a distance runner, but now exercising so hard will throw my body off in other ways and I don't feel good having pushed so hard. Perhaps this is exactly the point you're making, but I don't target zone 5 for long these days. Deep sleep, sure, but at what cost to the rest of my body?
All exercise causes fatigue and fatigue management is not really discussed but it's actually not that difficult. Most of your cardio should be done in zone 2 (look up the 80/20 rule). You also may need to start with easier workouts and work your way back up to what you expect you can do. I was also a former endurance athlete and in college I stopped exercising. Numerous times I kept on trying to get back into it and I didn't succeed at getting back into it until I realized that I needed to start from square 1.
I'm not fully back into it yet mind you, but I'm running 20 miles a week and lifting weights 6 days a week. A lot of my workouts are zone 2. I sleep at least 8 hours and I get roughly 1 gram of protein per pound of bodyweight from my diet (i also make sure all my other macro and micro nutrient needs are met).
The excessive fatigue goes away if you are meeting your nutrient and sleep requirements and not consuming drugs or alcohol. Look up the "repeated bout effect". I feel like a teenager again after getting my exercise, nutrition, and sleep routine locked in.
edit: To clarify what I mean about "the excessive fatigue goes away", when I first started running there would be mornings where I would wake up and have significant mental and physical fatigue and the very thought of going to exercise was repulsive to me. That goes away with time, it's also a signal to maybe take it easy for the day and do zone 2 cardio instead.
For sure, there is some kind of optimal balance to target. I've pushed myself so hard at times that I disrupted my sleep. For example, I bought a used Kickr and decided to do a FTP test at 9pm. This was not the smartest idea.
Running has been the goto for me. Can push myself on a 5k run in the evening (6pm) and when 10pm hits, I'm 2x more tired than normal.
And, depending on your situation, be careful. I was a collegiate athlete back in the day and recently tried to recapture some of that. The result? Torn labrum in my left hip that really can't be repaired (per doctor). I'm all for hard workouts, but respect your age (and prior injuries) if you're going to go down this path. Definitely ease your way into it.
