This is a great Bay Area route.
I like to get a ride out of the city up to Fairfax (take 101 north, Red Hill Rd. exit, over to Fairfax, then Bolinas road) to the Deer Valley Park. Here I go over Mt Tamalpais via any one of a variety of trails. My favorite is the high marsh trail to the CC Boys trail, but if you want to skip most of the climb you can go towards the ocean to the Cataract Trail. Then I head down into Muir Woods via Fern Creek, Panoramic, and Ocean Trails, where I get onto the Miwok Trail. Then through Tennessee valley to the Marincello fire road and to the SCA trail.
Finish at the bridge.
Any way you cut it, it's about 25 miles.