Road Riding Montauk is fun. The roads are friendly and you could easily hook up with a ton of riders out that way. You can also ride down E. Lake drive to Gin beach. Great view of the inlet from the opposite side. I think its about a 20-22 mile ride from the begining of town to the light house and back. Add a couple of miles if you go to Gin beach.
Hitherwoods Is an good trail. Definately will need a map. There are 2 trail heads. One at Navy beach and the other on 27a. Bring some insecticide
to fight off Ticks.
If you choose to go the MTB route and can break free for a little while.
Rocky Point, Glacier ridge, and setauket are great options. If you know somebody from the Island maybe they could hook you up with shoreham.
This By far the best trail on Long Island....but its not a marked trail.