It's time in the saddle on training rides that is going to count. Simply clocking up some miles is perhaps fools gold as riding up Mt Kenya is very little distance but a killer climb! You have to be quick to the 50 mile check point, which is where about 1/3 of the 203 riders failed. I'm also coming to the conclusion that time in the saddle on road rides is maybe the way to train effectively as there's no letting up on the cadence.
Don't just work on distance at the expemse of speed. Have some long days but don't go easy on your speed or you'll be sent back to Kielder at the 1/2 way point.
There are going to be a lot more riders this year, so that might get a little awkward at times – not sure quite how it'll work, but they'll have a solution. Book your accommodation if you haven't already! Consider that move to a 29er – many riders did very well on their 29ers last year (it looked like most of the UK's 29ers were lined up on the start line last September).
Perhaps go up there for a long w/end and ride the trails to better get a feel for the terrain and task? Either way, it's alreay become a legendry event and it's only its 2nd year in 2010!
Hope this helps.