Never done a 100 miler mountain bike race; but have done a 24 hour race that you do about the same mileage. For the 24 hour race on a team, your goal is to ride a bit faster than if you were doing it all alone; so you have to train for pace and intervals. But to ride a 100 miler mtn bike race, I would say your goal is to keep going and finish at a reasonable pace. So, I'd say just ride a lot; lots of miles, both on road and off. I say off road, cause as you know mtn biking takes more strength in upper body and arms and so you will have to have those areas of your body ready to take the bump punishment. I would not concentrate on speed or interval work as much as I would just miles, hills, time in the saddle kind of stuff.