Adjacent to the boardwalk is "the strand" a bike path that runs up and down the coast. The longest people usually take it is from Washingto Blvd by the Venice Pier all the way up to Will Rogers state park. Click the map image to open in Google maps:

You can go even further south and north on it than that, but that requires riding your bike in traffic in some spots. A comfortable ride that is also exercise is to ride a bike from the Venice Pier to the end of Santa Monica. Click the map image to open in Google maps: