When you park at the main trailhead, you'll see the entrance to the boardwalk as a closed gate (ostensibly so wild horses don't run out onto the road). All you do is follow the boardwalk to its end. About a quarter of the way, you'll see on the right a sign for the Maritime Forest Trail, which you can choose to take, or continue along the boardwalk. There's a very pretty lookout point at the end. When you're done, simply turn around and make your way back to the parking lot.