Why is there only 26 miles in a marathon?

Answer 1

26 miles was the distance the Greek runner covered, while running, to declare "Nike" after the last major sea battle during the Peloponnesian Wars. The Greeks begun honoring this distance as a 'marathon' distance.

Answer 2

A marathon is called so because of the greek myth where a messenger named Pheidippides ran 26.2 miles to deliver an important message during the battle of Marathon.