Ride your bike a lot, then ride it a lot more, then finish off with some more riding, then repeat. Then do it again the next day!
It's not about fitness as such, but endurace. I had a HRM and worked out what 75-80% effort was for me, then tried to stick to that. I'm not what you'd call that fit, but I finished when fitter riders were failing, becasue I put a lot of planning and preparation into myself, my bike, and my kit.
There's more to it than riding: Your bike need to FIT PERFECTLY. I played around with stem lenghts (10mm shorter than I was used to for a bit more comfort) saddles, grips, seatposts etc until I felt totally at ease. I played around with shock pressures a lot too.
Comfort is everything. Mose people were on FS bikes. I rode a Yeti 575 which was generally a burly set-up, but I borrowed some lighter wheels and tyres from Mrs PPs bike and it was just over 27lb all up. For me it was perfect. Very supple and comfy, but could take any hits that course could dish out.
There were plenty on HTs, but I wouldn't personally.
I serviced the bike front to back: Full NEW drivetrain & BB. Hubs serviced, shock and fork serviced. There was nothing to go wrong, and nothing did. During the week I checked the tyres once, fitted one new pair of pads and lubed the cables (2008 was also silly wet!)
Take as many spares and toold as you can in your kit bag, the on site service is expensive. I could have stripped my entire bike and replaced just about anything with what I fitted into a shoe box!
And, vitally important, learn what your body needs as fuel. I discoved I don't like too many chemicals (Gives me a bad stomach) and stuck to High 5 drink, Mule bars (natural energy bars) and a huge amount normal food. It worked for me.
DO NOT FORGET CHAMOIS CREAM! I used Udderly Smooth and it works, no doubt about it.
After that it's ALL IN YOR HEAD. Ever minute of every day for 6 days I was convinced I would fail. But I takled myself out of it and just kept turning the pedals and setting target after target ("Just get to the next tree/bend/feeding station") I thought that once I'd done 4 days I would feel better as I was over half way. And I did. Until the 5th day which was the biggest bitch I could imagine. It hurt like never before. Only on the last day did I let loose and up my pace. I blew myself apart that day, basically.
Was it good? Yeah. I reckon it was. An amazing experience and some super riding.
Would I do it again? Probably not. The investment in time and money make it a once-only thing for me I think.
Just DO IT!!