You will get cars on Highway One. Between Marshall and Tomales, there's often no shoulder, but the road is pretty twisty so cars generally can't get much speed up. (Motorcycles are a different story.) Around Bodega Bay on Highway, there's also a lot of traffic, especially RVs/trailers, etc., and again, not much shoulder.
A lot of the other roads out there are somewhat rough, but many have virtually no traffic on them.
I created
a 75-mile loop at Bikely.com. I'd ride it clockwise, i.e head north at the start. The main issue with it, other than being ten miles longer than you're looking for, is that there'd be few if any services between Valley Ford (the second time you pass through it) and the end, which is about 33 miles.
There'd be three stretches of extended climbing: Bay Hill Rd., Coleman Valley Rd., and Marshall-Petaluma Rd. And a lot of rollers in between.