I haven't done this run in a few years (it used to be one of my favorite vacation runs). Starting at the Salt Pond Visitor's Center on Route 6 in Eastham, follow the bike path that meanders through the woods (less than 2 miles I think) east to Coast Guard Beach. If you do this early enough in the morning you can catch a spectacular sunrise near the Coast Guard station house as you cross the wooden walking bridge over the salt marsh just before getting to the beach. Go past the Coast Guard house down to the beach and run north along the water (about a mile) until you get to Nauset Light Beach. You'll see the light house up on the left (it used to be closer to the bluff, but it had to be moved back several years ago due to erosion). Run up toward the light house and there's a road that heads west back toward Route 6. On the right hand side you can make a short detour to run past the Three Sisters Lighthouses (just in the woods on the north side of that road, not far from the beach). Follow that road for about 1 mile and bear right. Another half mile or so you'll intersect with the rail trail. Turn left (south) and follow that back to the intersection with Route 6 near the Visitor's Center. I don't think this run is more than 5 or 6 miles altoghter, but it's really nice, especially in the morning.