The mental thing is huge.
I haven't done a ride longer than 90 miles since May or June, and I haven't done a ride longer than 120 all year (I certainly haven't done anything over 130 since my last double century 3 years ago), but I did 170 miles of a tough double century 2 weeks ago. I'm not "fit" enough under any technical sense to do it, but I knew I could so I did. Plus I was helping a friend, and it was her goal event, not mine, so it was fun to see her succeed and take my mind off my own aches and pains.
I have also had at least one instance where I was fit as all get out, but just didn't want to keep going. (DMD 2009 - I DNF'ed at about 130 (?memory fades) even though I was quite a bit fitter and better trained than when I completed the whole event 2 years before. I knew how hard it was going to get and I just didn't have it in my head to keep going.)
