I was training for 6 hour race, but then one of my friends had to cancel, so we decided to just do it next year.

The training I was doing was lots of maybe 75% road riding (with one long ride on the weekend) to get my endurance up and the saddle time in. Then I would do a couple of mountain bike rides to keep up the handling and the technical skills and keep my body adjusted to the bike.

Sounds like fun! Be sure to report back.