Start at Natalie Greene Park in Ross. Small parking lot, its best to park at Ross Commons or near the Seminary. Take the route out from the parkin lot at Greene Park and up to Phoenix Lake, ride around on the right to a junction. The trail at left is the famous Eldridge Grade. Fish Gulch is the middle, a .75 mile steep climb, and the right trail is Shaver, this is a easy, sleepy trail up.
Eldridge Grade has it all in my opinion. From smooth packed fast fireroads running through redwoods to crusty, volleyball sized-obstacles to test your suspension. So what if its fireroad, go during the week and you will be amazed at the freedom you feel as you see San Francisco and the bay area at your feet. Buy a map at the many local bike shops and explore the area. I hook Eldridge up with many other trails to make a loop of some awesome, aerobic fun! The legal single tracks are in China Camp and at the Boyscout Camp in Tamarancho.
How to get there:
Highway 101 exit at Central San Rafael and head west to San Anselmo. Highway 101 exit at Sir Francis Drake and head west to Ross. Left at firestation to Park and parking.
Lots of people on trail on the weekend. Be courteous, most others are. Rangers will ticket speeders.