How long is the Boston marathon?

26.2 miles; all marathons are.

The reason is that the distance from the Plains of Marathon, the site of the battle in Greece, was 26.2 miles from downtown Athens. The Athenian citizens were preparing to evacuate the city via their navy. The Athenian army was trying to hold off the invading Persians. When the Athenians were victorious, the general dispatched a messenger named Pheidippides, who ran to the city, announced the victory, and died on the spot.