Unless something has changed, trails in Nisene Marks aren't open to bikes. The original owner of the land apparently made that a condition of turning it over to state parks. That said the trails are fun. It was actually a state park employee at the campground who told us to climb the fire road up to the 3-way intersection and look for a singletrack near a bench just beyond it. Then keep turning right. We ended up outside the park at a buddist monastery. You could also get dropped off to do your ride at Demo and then ride back over the ridge into Nisene, then ride back to town from there.