Hi, I love your screen name is it your real name too?. I love The Jerk. "I'm picking out a Thermos for you, not an ordinary Thermos for you, but the extra best.......ahem anyway. "He hates those cans!". Sorry.
It sounds like you've really been through it lately, well done to still be training. As far as what to do to feel young again? If I knew the answer to that I'd be a very rich man. But I think it's striking the right balance between intensity and workload and rest, but unfortunately it's easier to tell when you haven't got it right than when you have.
As you age you also have to admit to yourself that there is certain things you can't do at 37 that you could do at 20. I'm always deciding to do some plyometrics or depth jumps etc after having not done anything like that for ages and then I wonder why my achilles tendons hurt! Not to say you can't do them, you just have to start slowly.
Looking at your picture you seem to have a good amount of muscle there and don't have to far to go.
Do I spot a bit of "push your belly out a bit for the before photo" there?.