Fairfax Cyclery is conveniently located just below Camp Tamarancho, one of Marin County's last open access single track trails system. You can park at the store and ride up to some of the best single track in the area.
Camp Tamarancho Boy Scout Camp features a single 9-mile loop that's open to mountain bikes, almost entirely singletrack. Most of the riding areas around Fairfax are under the jurisdiction of either State Parks, Marin County Open Space, or Marin County Water District. Except Camp Tamarancho, owned by the Boy Scouts of America with access supervised by the Friends of Tamarancho.
One of the coolest things about Tamarancho is that bicycles are not allowed on the fire roads, only single track. Annual or day passes are required to use the camp.