Why is a marathon race exactly 26 miles and 385 yards long?

In 1908, the first modern Olympic marathon was designed to start at
Windsor Castle and end in front of the royal box in the London stadium,
a distance of exactly 26 miles, 385 yards, and that became the official distance.
The race honoured Pheidippedes, who in 490 BC had run 22 miles,
1,470 yards to carry news to Athens that the Greeks had defeated the
Persians on the plain of Marathon.