I don't know how you'd get from SFO up to Skyline, but I bet there's a way. And once up there I KNOW there are routes with bike lanes/bike paths that can take you down to the Great Highway, along the beach, up past Cliff House and around the headland through the Presidio to the bridge, and then once you're over the bridge ... hmmm. Well, I know you can bike up the Marin headland without being in traffic, but I bet others in the group can tell you ways to get further into Marin.
Oh, and there are any number of great lunch stops along the way. The Beach Chalet. Cliff House (if you can afford it and the line isn't too long). The cafe at the art museum up above the VA hospital (yum! And you can check out the art works while you're at it).