Give it a go, but you will waste your money of you don’t prepare. By prepare, I mean lots of long rides at weekends both on and off road, with lots of climbing, and a few mid week rides as well. If you are a bimbler, you might want to try the Kielder 50, or at least some local distance rides first. If you are a racer, you should be OK,just get your head into the right mindset – that’s more than half the battle.
You may also want to make sure your bike is matched to the job as well – well maintained is the key, after that it’s choosing the right bike (an 8″ travel DH monster might not be the best choice, but hardtails, FS, 29ers, 26ers, singlespeeds, geareds all make appearances – you just need a bike you are comfortable pedalling up hill and downhill for 12+ hours).
Good luck!