Bill Sycalik posted the challenge of running a marathon on this small island:

 I am on a full-time life experience project to run a marathon in all 59 U.S. National Parks (www.runningtheparks.com). I just completed marathon #38 since June 2016 on Garden Key at Dry Tortugas National Park in Florida. Garden Key is the second largest island in the Dry Tortugas, about 14 acres in size, and has a 0.6 mile trail around the outside of Fort Jefferson on the key. 

The Arabia Mountain Path is a paved, mixed-use path running 18.6 miles from the Mall at Stonecrest Trailhead to Monastery of the Holy Spirit, outside of Atlanta, GA.  There are several spurs as well.

Robert Rives posted the route:

The Neuse River Trail, as described on Wikipedia: