I know this is a little late, and hopefully you've had the opportunity to go and check it out for yourself by now, but just my .02 for one trip. I went up there for a day hike and got to check out a few of the bike trails. Most of the roads are pretty gravelly and some stretches of decently packed dirt. I probably wouldn't take a road out there but an MTB would be a blast. Some spots are decently hilly so at least it can be a bit of a workout depending on the route. There's also quite a bit of distance available if you're looking for it. I believe I read something like 100 miles of total road (both gravel and dirt).
Good luck and safe riding