I did it geared last year, 4 hours 50 minutes. When I got to the first climb, I was the only person I saw who cleaned the climb up the road and up the off road to the top – everyone was pushing. It was a first and second gear crawl most of the way and required a lot of shouts of '_RIDER_ coming through'. The last big climb, everyone walked. It's not impossible to single speed it, but you'll enjoy it more geared and with a bouncy fork.
Didn't get in this year. Will be applying to get in 2011 race at 9.05am on Sunday, seeing how quickly it filled up this year.
Good luck! Fit a rear mudguard.