The longest ride I can remember doing was 112 mi in a triathlon in Japan. My time was something like 5hrs 10min (unloaded, of course).

I've toyed around with the idea of doing a 24hr MTB race, but I don't think there any around here. They have a big one in Tucson where I used to live, but maybe it's for the best that I don't get mixed up with that!