Marathon is a city on 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 recorded by the U.S. Census Bureau is 10,626. Marathon apartments for rent comparison page shows most up to date rent information on Marathon apartments and houses. Rent information is obtained from multiple rental sources but we cannot guarantee their accuracy.