Originally Posted by BlastRadius
Off road? Cyclocross? Mixed-terrain?
Yes, mixed terrain. The route starts out on the road, crosses the bridge, then veers off the Sausalito bike path at the school there and cuts through some Mill Valley neighborhoods using Sycamore Rd. Eventually, you end up at the trail-head for the Railroad Grade fire road.
Railroad Grade goes all the way to the top of Mt. Tam (East Peak) without ever getting steep. It was recently graded too, so while it's not paved, it's virtually the same as riding on a paved road these days. There are the occasional patches of rock or gravel, but they are few and far between. I'd recommend 700 x 25c or above tires -- I use 700 x 28c -- but I think it's doable even on 700 x 23c's.
Once we're at the top, the rest of the route is paved as we do the Seven Sisters in reverse, then go to Fairfax, then return home. I think it ends up being about 55 miles or so.