Marathon is a city located in the U.S. state of Florida. It is within Monroe County on Knight's Key, Boot Key, Key Vaca, Fat Deer Key, Long Point Key, Crawl Key and Grassy Key islands in the heart of the Florida Keys. It has a total area of 9.27 square miles at 24°43′35″N 81°2′25″W. The elevation is 3 feet above sea level. As of the 2010 census data, it had a total population of 8,297. The population density was 1,055.44/sq mi. It has Zip codes 33050 to 33052.