There are many places to hike within a hour (or less) of SF. Here is a good one:

http://www.openspace.org/preserves/pr_purisima.asp

There are also many other preserves listed on this same site.

Muir woods is great, and you should check it out. But it's very heavily vistied, mom's with strollers, grandparents, etc. Unless you get up into the higher elevation trails.

The link I gave you, is a more remote, much less heavily visited area.