We see a lot of the Blazing Saddles people over our way. They cross the Golden Gate bridge, and come to Mill Valley and decide they're going to ride to Muir Woods. I always find them about a half mile up what's probably a 2 - 3 mile climb (with some double digit percentage grades), stopped at the side of the road, looking at maps and scratching their helmets. I always stop and give some direction (like don't take Wildomar north of Molino, the street with the 25% grade because I don't even like to drive up it). My daughter often gives them advice on her way home from school - we were just talking about this today.

If you do want to come north of the city, there are some nice loops in Marin and a great bike path in Sausalito. Just know that Muir Woods is a pretty technical climb/descent.