The Greeks didn't run marathons. The marathon is a purely modern race, defined as the distance between the marker at Marathon and the finish line of the Panathenaic Stadium at Athens, and run for the first time on April 10, 1896 at the first modern Olympics at Athens. The Greeks measured plenty of other distances with a variety of surveying tools (the simplest of which, simple yardsticks, were known since at least the Egyptians) but the distance of a marathon race was not one of them