I have runs occasionally where i can run for as long as i want and the pace gets quicker and quicker, effort feels like i'm flying; but in between these days (which is most of the time) i get tired like crazy, no energy, just crashing and burning. i can run 10 miles in 7 min pace and then another day i'll be running 8-9 min miles for 3 miles and then i have to stop and walk. It doesn't make any sense, i've been doing easy progression runs and never pushing myself, most days i just get extremely tired and have to walk. I don't fit many of the symptoms for overtraining, seeing as how i eat like a horse and sleep fine. Just wondering if anyone knows what could be the reason for this, thanks!