You can do a nice loop south of Duck, a combo of designated "bike routes" and connector roads. Kitty Hawk cycles on Eckner St. in Kitty Hawk has the "bike route" map, Micheal the owner can show you the loop. It has some very modest hills (which I like), nice views of the sound, an ocean front stretch, etc. Riding from duck, doing the loop, and back to Duck should be about 30 miles.