Everyone is ignorant in some way though, you cannot know it all.

The full story is that before the Battle of Marathon, Pheidippides ran around 240 km to Sparta to ask for aid in the upcoming battle and then back to bring their message. He then fought a probably exhaustive battle and then ran to Athens (if I am not mistaken) to tell them of the victory, adding ~ 42 km more.

So in total he ran around 524 km. Not sure in what time exactly, but I assume he wasn't tardy. Then he died of exhaustion.

By the way, the run to Sparta is, since a few RAF officers did it after WW2, now also a race and your name gets put on a slab of rock somewhere.

That's because the legend is most likely a mix of fictional accounts. The one you are thinking of is the story recounted by Lucian of Samosata around 600 years after the events and is a romanticized retelling of a story by Plutarch from a 100 years before.

The one the person who responded to you claiming that he ran like 500km is an even more fictional account from 1874. From an author that combined stories from 3 different more contemporary but still likely fictional accounts.

So honestly you are more correct than the vast majority of the people on this thread.