I have done that Beltparkway-Crossbay-rockaway loop quite often in the past, but the greenway path between Rockaway pkwy and Crossbay blvd is horrible and an easy place for flats. Had to do the loop last weekend while Prospect Park was being paved. Luckily only 1 flat during both days.
To make that loop longer you can go into Breezy point.. an extra 10 miles total.. Or you can go east on beach channel into Atlantic beach and out to Lookout Point.. Roads are okay..