In reply to Andrew W:
Road or off-road ? Have done the latter, but as part of an event.
I had a can of red bull an hour before the start - woke me up and cleared me out!
Shot blocs kept me going - I'm referring to my legs here - but if time allows try and include a couple of cake stops - variety is good.
Try and front load your climbs, you want to be able to tell yourself at 50 miles that you've done the worst of them.
Make sure you stretch your back out and relieve the pressure on your arse every 30 mins or so. Easy to forget when you get going.
Most importantly you need to be enjoying it, if you haven't got a small on your face after 25 miles then you probably be really loathing it by the second half. I made sure I enjoyed the views and the terrain, and kept telling myself I was having fun. By the end I was so tired that I even believed it !