working out in my 40s just makes me tired. used to energize me when i was younger.
I have a feeling you are a young person :)
If you haven't tried a creatine supplement, I'd suggest trying it. 5g/day makes a huge difference for me in workout recovery.
Just wait till you're in your late 70s...
I'm funnily enough 40. You can definitely overdo exercise, but going from zero to some is generally very good for energy in my experience. It generally lets you turn that sort of tiredness you can't rest yourself out of into the sort of tiredness you can.