It's a point, so not many road choices, but generally Montauk Hwy (State Rt 27), the main east-west road is a good cycling route in any direction, from Montauk Point west. A side trip is Old West Lake Shore Rd. up to Montauk Harbor, then Flamingo/Edgemere back to town. Then west for as far as you desire.
One loop from the traffic circle (center of "downtown" Montauk) out to the point, then back to Old West Lakeshore, up to the harbor, down Edgemere to town, west on the "new" Montauk Hwy" (Rt27) to Hither Hills State Park, then back to town via "Old" Montauk Hwy (it's closer to the water), would be 24 miles.
If you wanted to extend, continue west on MH to Napeaque Rd, then it becomes Cranberry Hole, then back onto MH in Amagansett, and east and back to town, with a turn at Hither Hills onto Old MH. That's 35 miles. Or in Amagansett, continue west on MH/Rt27 to wherever you want to turn around.
Look online for a GPS log/map of the "Tour of the Hamptons". That's the big organized done out here in late Sept. Assorted routes from Southampton, including the century that goes to the point and back.
Once you hit the Amagansett, there's ton's of roads north of Rt 27, as well as few quiet roads south of 27 as well, some of those dead-end at the beach.
MapMyRides has some info. here: http://www.mapmyride.com/us/montauk-ny/
Last edited by Steve B.; 08-18-13 at 03:02 PM.