In my experience, that probably is enough time, unless you’re interested in doing some serious hiking. There’s a loop trail which gives a nice idea of what the woods are like, and from what I remember it can easily be hiked in that amount of time. On the other hand, I’m sure it would be possible to spend more time there – or, better still, also take the time to go up to the top of nearby Mount Tamapais (you can drive most of the way up the mountain, and this time of year, the hike to the very top will be a breeze). The views from up there are gorgeous.

So, if you don’t mind driving in an unfamiliar city (I don’t find SF particularly daunting, except that there’s a lot of traffic, especially at certain times of the day), it might be worth it to rent a car and see Muir Woods and Mount Tam. (Unless you can get a tour that will take you both places). As for Muir Woods itself, though, if you’d rather not drive, I think it can be seen in the time they’re giving you on the tour.