In 1908 the Summer Olympic Games were meant to take place in Rome, but the Italian government was experiencing some financial difficulties. The Games were thus awarded to the city of London!
The marathon started at Windsor Castle so that the royal family could get a good view from the castle.
It finished in The Great Stadium, which had been built in White City, west London, to host the Olympics.
This distance from Windsor Castle to White City is 26 miles (42.195km), and this became the official distance for the modern marathon.