Marathon is a city on Knight's Key, Boot Key, Key Vaca, Fat Deer Key, Long Point Key, Crawl Key and Grassy Key islands in the middle Florida Keys, in Monroe County, Florida, in the United States. As of the 2000 census, the city had a total population of 10,255. As of 2005, the population estimated by the U.S. Census Bureau is 10,626.
Marathon is located at 24°43′35″N 81°02′25″W / 24.726310°N 81.040327°W / 24.726310; -81.040327.
According to the United States Census Bureau, the city has a total area of 9.6 square miles (25.0 km²), of which 8.6 square miles (22.4 km²) are land and 1.0 square miles (2.6 km²) (10.37%) are water. Its city limits extend 1200' from land into the adjacent waters.