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 of the Florida Keys, in Monroe County, Florida, United States. As of the 2000 census, the city had a total population of 10,255. As of 2014, the population estimated by the U.S. Census Bureau was 8,708.HistoryThough the area has been settled for some time, Marathon is a relatively new city, incorporated in 1999. The city's boundaries (according to both the city and a 2001 Rand McNally road map of the Keys) extend from the east end of the Seven Mile Bridge (Mile Marker 47) to the west end of Tom's Harbor Bridge (Mile Marker 61), excluding that portion of the area within the city limits of Key Colony Beach. Among the islands found within the city limits are Boot Key, Knight's Key, Hog Key, Vaca Key, Stirrup Key, Crawl Key, Little Crawl Key, East and West Sister’s Island, Deer Key, Fat Deer Key (excluding the portion in Key Colony Beach), Long Point Key, and Grassy Key.