This ride is definitely an adventure.  It is perfect for a cross bike, or if you want to ride your mountain bike, pump the tires way up or get some narrower tires.  I did this ride yesterday with Bill, and thank goodness he rode his mountain bike.  The ride is probably around 80% on road, but it has some great singletrack in the middle which is relatively smooth compared to Annadel State Park.  Make sure you are prepared for anything out there, because it is quite remote.  Last time Aaron and I did this loop, he managed to rip his rear derailleur off part way through and we had to cut his chain and he rode home on a singlespeed.  We met Claire in Guerneville for some great Mexican food.