Are you looking for other marked, narrow, designated bike trails, similar to what you've done, or are you open to unmarked sand roads as well? If the later, your distances options are endless, given a park brochure, printed topo map or a phone/gps. I prefer that type of riding myself, exploring new areas rather my tires pounding the same sand so many others have. I've ridden Brendan Byrne State Forest quite a few times. Most of the roads are on the park's trail brochure, in addition to the marked trails, with nice views of old cranberry bogs and cedar swamps. Wharton has even more mileage to explore. I've ridden the Penns Branch Trail but most of the other guys were riding hard and fast, and seemingly not paying attention to what was around them. Great if that's what you are into but it's not my thing.
Wells Mills County Park, in Waretown, has a 6.2 mile multi use trail, which is part narrow trail and part old sand road. It's fairly well-marked. I know the guy that marks it.