I did an 8 miler on the treadmill today at 11 am. I skipped eating anything from dinner last night (fish, veggies) at 6 pm until today at 11, which was a long time after the run yesterday. I was definitely feeling weakness during the whole run. It's a weird feeling - you are tired, but your HR is low (like 120). I tried speeding up and that sort of worked, but turning of the book on tape and switching over to music really helped.
8 miles, 8 min/mile average (7.5 mph), 121 HR average