Depending on where you are in Novato, head west out of town on Novato Boulevard, or west out of town on San Marin Boulevard (which hooks up with Novato Boulevard), and this will lead you to the glories of West Marin. Grab a map of Marin, and you'll get a quick idea of where you might go.
One classic ride from there is the "Marshall Wall": Novato Blvd, right on Petaluma-Pt Reyes Rd, left on Hicks Valley, left on Marshall-Petaluma road, left on hightway 1, left on Petaluma-Pt. Reyes Road, return Novato Boulevard. The crux of the ride, as the name suggests, is the Marshall Wall, which is a good climb but worth the effort. Great scenery, rolling country roads through open fields, and if you catch the tailwind on Highway 1, nothing beats the ride along Tomales Bay. (I do this ride from a slightly different start point, Lucas Valley, so I don't have the exact metrics, but I would guess about 50 miles, and maybe 3000' to 3500' of climbing. Perhaps our Novato neighbors have better figures.)
Lots of other options and loops in the West Marin area, but I will let the others chime in with their suggestions. Hope you have a great visit.
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