A couple photos here
(with a story about last year's ride and the event in general).
I was never interested in doing it because I didn't have the right kind of bike (and was doing much less biking). You can ride most of the roads on a road bike or a mountain bike but it's a lot better on something in the middle.
The roads vary a lot. A lot of the time they're very good -- hard packed, generally the best riding is where car wheels have gone, smoother, better traction. Actually better after some rain, packs things down more. After dry weather the surface can get loose, less traction going up and less stable going down.
And then, of course, sometimes not so good. Sometimes the dirt is too loose, some places get muddy (as opposed to well packed) after rain, there can be erosion issues after a lot of rain, there can be washboard in places, or the road can be not maintained, and who knows what you get.
More difficult than road riding? Sure. But virtually no traffic, lots of variety, beautiful country. And a bit of difficulty adds to the adventure.
And certainly a great cause to support.