People warned me about doing the century on my MTB but I felt fine on it. What got me was 2 weeks later when I went to redo a 60 mile section with my gf. I ended up wearing a pair of regular shorts and boxers and it started to rub my butt raw so I had to adjust my riding position a little and that's when the pain started. I suppose I could take a week off to let it heal but I just can't stand getting around by car or subway anymore.
Regarding the fore-aft position, I currently have my seat so that it's slighty behind and the nose dipped slightly forward cause recently I started doing some out of the seat mashing. Maybe that's what caused it but I doubt it cause it all started right after the 60 mile ride with my gf.