Short out-and-back trip (about a mile each way). It's fun to do laps of this one for training or cardio purposes. Mostly fire-road-style with singletrack at the end. You're not allowed to bike past the bridge where it turns into sand/beach, so bring a lock and a lunch and hike out the remainder of the way to the ocean.
From US 101 in Marin County, exit San Anselmo (Sir Francis Drake). Drive about 20 miles west on Sir Francis Drake. Turn right onto Highway 1, then after about 0.1 mile, make the first left onto Bear Valley Road. Drive about 2 miles to the junction with Sir Francis Drake, and turn left. Drive about 5 miles to the junction with Pierce Point Road, and turn right. Continue about 3 miles more to the trailhead on the left side of the road.
http://www.singletracks.com/item.php?c=1&i=3728