Strangely some of the trails on the map that look to be suitable for off road mountain biking are marked on local signs as not for bikes. These trails start out as suitable but change to steep, narrow paths only really useful for hikers. I went from the hire shop on Columbus Av. down to Fisherman's wharf, past the bay side marinas then up to the Golden Gate bridge. After the bridge crossing, currently restricted by repair work to a crowded two way bike and people path on the west side, I turned left and climbed the steep hill road to the costal artillery battery emplacements. Beyond there past Rodeo beach the real country hills begin. After nearly getting stuck on Wolf ridge, I headed toward to Sulsalito and then along the ridge back toward the bridge and city beyond. It was only 29 miles but certainly some of the most satisfying off road riding to be found.
There is lots of information on the net about mountain biking in Marin county and reasonable quality bikes can be reserved and hired from Blazing Saddles in Columbus Ave. If you intend to go off road be sure to get a proper Marin bike rather than just one of the regular tourist hack bikes also available from this and other nearby shops.