I used to be that way...from late 20s to mid 30s, every run was a drag. Always tired, with something almost always hurting. I was seemingly always battling, resting from, or coming back from an injury.
Then in my late 30s, with the help of a physical therapist, I discovered the gym. It has been the fountain of youth for me. Four times a week, I hit the gym for weights and the stationary bike.
I replaced some of my running miles with gym work. I know that may cause some heads to explode, but hey, I wasn't PRing anymore anyway.
Now in my 40s, I run about 40 miles a week, plus the gym work, and I feel better than I did in my 30s when I was still trying to grind out 80+ miles a week with no supplemental exercise. Of course I'm slower, due mainly to age, but not by as much as you'd think. But the main benefit is that I actually feel good while running again, instead of just dragging my broken down body through another run.
The gym fixed me. I honestly feel better in my mid 40s than I did in my mid 30s.