When did pheidippides invent the marathon?

Answer 1

There is no actual proof that this occurred at all. Many say that his running from Marathon to Athens was only partly based in truth.

But if it did occur at all it would have been in the same year as The Battle of Marathon, which occurred in 490 BC.

Answer 2

It was before the battle - he was sent to summon the Spartans to join in. They arrived too late, as the Athenians were forced to engage the Persians before they arrived.