Why Is A Marathon 26 Miles?

From the "Word Power" section of the April issue of Reader's Digest:

A marathon is 26 miles and 385 yards. The word and that oddly specific distance date back to the 490 BC battle of Marathon, Greece, in which the Greeks defeated the Persians. A messenger carried news of the victory to Athens across some 26 miles. Centuries later, in the 1896 Olympics, the footrace debuted and adopted the name marathon in honor of the runner.

Is anyone else wondering how long it took that messenger to carry the message? :)