I've been running for a few years now, since 2000. Not always consistently, by any means, but over time, I have some decent miles on my legs and muscle memory can take over where my training leaves off.
But when marathon training season comes around, I start having the same doubts every time, i.e. "What the hell am I doing this for?" or "I hate running long!" or "Am I going to be able to finish this long run?"
Clearly, after 4 marathons, I can go the distance. I just find it strange that sometimes my mind has more power than my body - or actually, that I GIVE my mind the power over my body. So maybe it's just a matter of "don't think, just run."