I also do 1 hour aerobic workouts 2X a week at 7:30-7:50 pace.
At this point, I'm absolutely convinced it's either my ferritin or iron/hemoglobin. I'm going to get blood tested Friday afternoon, so I can find out.
I heard normal levels for Serum ferritin is 18-270 for sedentary males but 25+ (Ideally 50) for runners, with hemoglobin at 13-18.
(Hemocrit at 44% or greater)