In 490 BCE, the epic battle of Marathon took place in Greece. 600 Persian ships invaded and delivered a massive army to battle 11,000 Athenians. The Athenians proceeded to beat back the Persian army at Marathon, then marched all the way back to Athens in full armor, where they blocked a second Persian army from invasion.
In the meantime, legend has it that the Athenian soldier Pheidippides ran back to Athens to alert his countrymen of the victory at Marathon, then dropped dead from exhaustion. There is debate about whether Pheidippides actually made that run, but it is clear that the Athenian army did march back, in full armor, roughly 25 miles, after fighting for hours—quite a feat! The Olympic marathon event is based on this bit of history, though details like the length of the trek have changed a bit over the years.
Here's a detailed explanation of the story of Marathon, including a bunch of cheesy CGI animation showing how the soldiers were arranged. If you can get past the circa-2004 animation, this is really fascinating military history. Enjoy:
(Via David Brin, in an entertaining rant about Frank Miller's 300.)