There's a bike path that takes you the entire length of San Buenaventura State Beach. Officially it is called the Omer Rains Trail, named after California State Senator Omer Rains, who served the Ventura County area from 1974 to 1983.
While I don't have the specific start-end points of the Omer Rains Trail, I do know that it's a lot of fun taking the bikes to Marina Park at the south end of San Buenaventura State Beach and taking the bike lane on Pierpont Blvd about a mile to where the beachfront bike path starts at the corner of Pierpont and San Pedro St.
Great bike path along the beach in Ventura! Nice and flat with cool ocean breezes, with several restrooms available.
From there, the two-lane bike meanders along the beach to the Ventura Pier and up the Ventura Promenade to Surfers' Point. On weekends it can get a little crowded around here with pedestrians, other cyclists and beachgoers, so do be careful.
From Surfers' Point, you can continue taking the bike path north/northwest to Emma Wood State Beach, cycling around the Ventura Beach RV Resort, past the City of Ventura's Seaside Wilderness Park, then on up the coast. Or if you want, veer right on Main Street and make your way over to the Ventura River Trail that takes you up to Ojai.
Or just turn back and enjoy the views, perhaps stopping and having lunch on a bench overlooking the ocean. Or make your way up to Downtown Ventura and have a bite.