Here's the official description:
PICTURE THIS: A 199 mile running event that will push you and 11 pals as far south as you can go in the continental U.S. Starting in Miami and finishing in Key West this relay offers runners a unique opportunity to run, near the ocean side, and across the southern end of the state of Florida. Along the way you will run across multiple bridges including the Seven Mile Bridge hopping from Key to Key, passing through popular destinations such as Key Largo, Islamorada, Marathon Key and Big Pine Key. The race (and your stamina) will wrap up in the tropics of the southern most city in the continental U.S.A., Key West.
If this video won't play here, click on it to watch on youtube - it does a great job of showing why people enjoy it so much - it might even make you want to try it too!