Why is the marathon 42km long?

Legend has it that Pheidippides, a soldier in the Greek military, ran from Marathon to Athens with the news that the Greek military had defeated the Persians in the Battle of Marathon. Supposedly Pheidippides collapsed and died after giving the news. No documentation has been discovered concerning this and it is really not known whether this actually happened or whether it is true legend/folklore. The distance between Marathon and Athens was around 40 km.