Just came back from a SF > Pt. Reyes (actually, Olema) > Mt. Tam inn to inn that was awesome. I did it on my cyclocross bike with a light load. I did true mixed terrain, but I think if you are on a mountain bike, you could add a lot more dirt.

I started with the standard route out to Fairfax and then Pt. Reyes on the first day (could have added dirt on the way to Fairfax and through Henry Coe). I then stayed two nights at the Bear Valley Inn (10% off arriving by bike, 7% more off for LAB members), which had secure indoor bike parking. I hiked instead of riding in Pt. Reyes, but there are lots of trails open to bikes. I then headed to the West Point Inn on Tam (bike or hike in only; hot water and modern kitchen, but no electricity) by going down 1 to the Randall Trail, then Bolinas Ridge Trail to Ridgecrest Rd., and finally, Gravity Train Rd (a fire road). The proprietor of Black Mountain Cycles recommended Randall over the northern part of Bolinas Ridge due to its shade and superior traction (I was on 32 mm slicks). While it's a lot steeper on the map, it didn't seem like it, probably due to the great traction in the rain forest.

The southern Bolinas Ridge Trail was the highlight of the the trip, with huge redwoods that shaded the trail so much that I was actually riding through mud in parts, even though it hasn't rained for months. The last day I just descended Gravity Train (five miles) into Mill Valley and took the road home, but if I had more time, I could have easily made most of the route dirt.

If you get the Marin biking map, you can see all the trails possibilities.