Here is the ride I would do. Pretty hilly, but gorgeous ride.
Head east on Sir Francis Drake to San Geronimo.
Left on Nicasio Valley Road at Golf Course.
Ride over Golf Course Hill into, and then through, Nicasio
Right on Petaluma-Pt. Reyes Road
Climb Hill to Cheese Factory, then slightly past
Turn left on Hicks Valley/Wilson Hill Road
Left on Petaluma/Marshall Road
Over Marshall Wall, then down to Hwy 1
Left on Hwy 1. Ride along Tomales Bay to Pt. Reyes Station
Get an almond croissant at the Bovine Bakery and refill bottles
South on Hwy 1 out of town, then right turn on Sir Francis Drake towards Lighthouse
Ride SFD through Inverness and all the way to the end at the lighthouse.
Retrace your route back towards Inverness, then turn right on Bear Valley Rd.
Rejoin Hwy 1 in Olema, then a quick left on Sir Francis Drake
Short climb over Olema Hill.
At Platform Bridge Road (bottom of the hill), look for the bike path. You can ride a short dirt section on the right side of the road, or go under the road from the opposite side where P.Bridge hits SFD. (I would avoid SFD through this section. It sucks).
Ride the bike path back to your campsite.
This one sounds very nice. Another option if you want a major climb would be to ride South on hwy 1, turn left before Stinson Beach to climb Mt. Tam, retrace back to go down on Alpine Lake Rd to Fairfax and head back West on SFD to the campsite.
The view from Mt. Tam over Stinson Beach is to die for.
Pegoretti, Merckx (stolen by a murderer), Colnago, Ducati 696
Those would be great rides.
You could also go over Olema hill and then north on 1 toward Valley Ford. From there, you could do a great loop through Freestone/Occidental, over Coleman Valley Rd and back down Hwy 1 through Bodega Bay.
I am a google maps idiot so I can't draw it out til I get home to a computer with topo, but I expect that would be about 90 miles, maybe a little more.
If you're looking to go 80 - 100 miles, going up to Bodega Bay is a good option. Specifically, you can climb Coleman Valley Road, which is pretty challenging and also has awesome views of the ocean.
From SP Taylor, you could take the bike path until it ends at Platform Bridge. From there, here's a 95 mile loop, chosen for low-traveled roads. Most of this route is used in the Mt. Tam Double. There are a few stretches where there is more traffic and/or less shoulder, but all in all, it's a pretty bike-friendly route.
There will be stretches that are somewhat remote and without a lot of services. Strong winds can sometimes be a factor too.