After a couple years of up-and-down training, I finally decided it was not in my head and had blood work done. I was diagnosed with anemia. Hematocrit 38, hemoglobin 13.7 I think, ferretin 20. Other iron numbers that I don't recall the name of were worse.
By the time this test was done, I had gone from 80-90 mpw down to less than 40, and even an easy 6-miler would tire me out. Running 10+ has been nearly impossible at sub-7 pace (I'm a low-15 5k when in shape).
The iron has come up with supplements a little bit over 8 weeks, HCT to 40, hemoglobin to 13.9, and ferretin to 42. However I have still gotten worse running-wise to the point where I'm just going to stop until I figure out what is going on. It used to be that a couple days off would leave the legs refreshed and light-feeling, but even a slow run feels like an effort and causes heavy legs now.
Has anyone else had a similar experience? What did your anemia do to you? I've had over-training before and this is certainly not that (no elevated resting HR, no insomnia).