Build up gradually but remember you are on a Mountain bike not a road one. Don't try to stay with the roadies. I do randonnees and do a couple of metric 100's for training for my offroad 100. then there's the 100miler at the end of the year on the road. Doesn't matter what pace you go at, but make it easy on yourself. Get slicks on- but don't go too wide (Or if your on a Hoss too narrow) I use Schwalbe city slick in 26x1.4 and the advantage of these is that they will take 110psi. Far better than some Mountain slicks that will only take 80psi. They roll well and will take a few knocks so are not too fragile.
If you are doing 30 now, then do it non stop. then up to 40- then 50miles. Unless I am on a Randonnee, I don't bother going above this distance for training. Other thing is eat a good breakfast, eat on the rides and drink and drink and drink. Just to give you an idea. My 100mile off road will take 12 hours. In the 12 hours, me and Stuart will drink 25 litres of water. Me and Stuart? Yea, it's that bloody Tandem team again. Don't knock Tandems- Just try to catch them.