Answers.com claims that you need to stop believing the myth that marathons are 26 miles long “because of the ancient Greeks”:
One common myth is that marathon is 26 miles because that is the length that the Greek messenger ran from Marathon to Athens to announce a Greek victory.
In actuality, today’s race length dates back to the 1908 London Olympics.
Runners were set to race about 26 miles, but an extra 385 yards was tacked on so that the royal family could good view of the race, according to the NY Times.
Google Maps claims that the shortest route on drivable roads between Marathonas and Athens is 42.7 km, which translates to 26.5 miles, so it doesn’t sound like too much of a myth. Wikipedia says that:
The International Olympic Committee agreed in 1907 that the distance for the 1908 London Olympic marathon would be about 25 miles or 40 kilometres. The organisers decided on a course of 26 miles from the start at Windsor Castle to the royal entrance to the White City Stadium, followed by a lap (586 yards, 2 feet; 536 m) of the track, finishing in front of the Royal Box. The course was later altered to use a different entrance to the stadium, followed by a partial lap of 385 yards to the same finish.
In other words, prior to the 1920s, there was no standard length for a marathon (just as there is no standard size and shape for a baseball field even today). They then settled on one based on the 1908 Olympics, which only tangentially had to do with the royal family.
A better myth that you need to stop believing would be that Pheidippides ever made the run in the first place, following the Athenian victory over the Persians in the Battle of Marathon in 490 BC. Herodotus, our main source for the Persian wars, mentions a runner named Pheidippides who ran to Sparta to ask for help, but the first mention that Pheidippides ran from Marathon to Athens to announce “Victory!” right before falling down dead was Lucian of Samosta, who lived and wrote in the second century AD. In other words, like Archimedes shouting “Eureka” and running down the street naked, or like Newton getting hit on the head by an apple, it’s one of those delightful stories that add spice to a lecture, but which must then be disavowed. (Stephanie Trigg would call it “mythic capital.”)