Marathon is a city of Florida in the United States. It is 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. 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.