Head up Hwy. 1 to the campground amongst the trees, ride across the road, up the gravel, the off into the hiking trails. Stay on Hwy 1 as it snakes. Follow signs to Mt. Tam.
Road eventually forks.
Follow the arrow to Mt. Tam. Avoid the Muir Woods area at all cost. Don't even think about hitting the trails here at Muir. Go about 5 miles. Park with all the hikers on the left.