These past few meets I've felt like crap at about half way through the race (8k). I don't have any power at all, and I have to really jog on the hills. My repeat miles on Tuesday are really quick. Last Tuesday I did a workout on the track that went like this: 4x1600 in 4:59, 4:57, 4:55, 4:55 with three mins rest (lap jog+a minute). Then, I went to a race on Saturday, started out strong (10:40) then ran out of power and ran a 30:09. What could be my problem?