This may not be to everyones taste but I just completed my first mountain bike marathon yesterday. The Ruthin round of Meridas mountain bike marathons.
93 km of fantastic riding, must be 80-90% off-road (bit of road needed if you are joining that many trails I guess) people waiting to feed and water you at a couple of points around the course, fantastic atmosphere with chatty other riders. 3000 meters of climbing did take a little getting used to though.
Being at the back (well, dead last as it happens, but it was my 1st one) I also got the full benefit of the quads, motorbikes and vans that constantly sweep the course as well as the fixed marshalling posts and was never once asked if I wanted a lift, shortcut or otherwise wanted to bail out, just thumbs up and "you OK? Keep going mate well done" all the way.
All round one of the very best days out I have had on my bike and a HUGE sense of achievement to boot. Thoroughly recommended to anyone with any kind of off road enjoyment. They do 25, 50 and 75 km rides at their events as well as the (approximately) 100's.
Tracker