This is 68 miles with 8500 feet of climbing and a boatload of fun. You'll need minimum 35mm tires or you'll be having too much fun and can forget about getting home before dark. I'm running 43mm Rock n Road tires which are great for this ride.
It's reasonable to plan on about 10-11 mph average rolling speed and one hour for stops. This means about 7 - 7'5 hours total for the ride.
We will stop for two minutes at West Point Inn to refill water, and then at Lagunitas for a quick lunch. Apart from that we'll need to keep moving.
We take a fast'n'fun route up to West Point, using Pixie Trail, then Pipeline. Laurel Dell is beautiful and Bolinas Ridge is an all-time classic. San Geronimo Ridge is very hard, but it doesn't last forever and at the end awaits a delightful circuit of the Tamarancho singletrack. Then we drop into Fairfax and cruise home on asphalt so as to be back before dark. Also, 8500 feet climbing is enough for one ride.
Bring some lights just in case we have a mechanical and end up riding home after dark.
Due to daylight constraints, we must roll at 9 a.m. sharp from Sports Basement.
Caveat: If you're not (a) very strong and in great shape, and (b) very experienced at road and mountain biking this will be a horrible day for you. But if you are both of the above, it will be one of your best rides of the year.