I did my first 100 miler this year. I didn't have a lot of time to train but I have a lot of base so that seemed to work out OK.

I'd say that the most important thing is to get in some long back to back days on the bike on the weekend. I did a lot of my endurance training on the raod bike then I worked a lot on efficient mountain biking on shorter rides. On a long day like that, efficiency is key. Also, try to learn how to minimize your time of the bike, even 15 min farting around at an aid station will lengthen your day substanially.