Yes I am also interested in doing long, marathon, off road races/tours.
At 42 I have no desire for jumps, drops, and other stunts. I have been riding my mtb on the road with slicks for exercise but am looking to go off road more. I have put the knobbies back on the bike and am just trying to steal time to get off road.

I looked into the Vermont 50 (http://www.vermont50.com/) but I believe they reached their limit and have closed registration. I do plan to do it next year though.

There are other events that I am aware of though that you might be interested in.

Wild 100 http://www.ertc.com/Wild100.cfm West Virginia

I can't think of any others right now but check around...there's plenty

Good Luck