Pheidippides didn’t run a marathon. He ran 260 km over two days and died. A modern marathon is “only” 42.195 km.
You’d probably die as well without training for said marathon, which that poor man didn’t have the luxury of doing.
Oh man, are you selling it short. He was a professional running-courier, so we can assume he was well-seasoned for the activity, BUT
The traditional story relates that Pheidippides (530–490 BC), an Athenian herald, or hemerodrome[3] (translated as ‘day-runner’,[4] ‘courier’,[5][6] ‘professional-running courier’[3] or ‘day-long runner’[7]), was sent to Sparta to request help when the Persians landed at Marathon, Greece. He ran about 240 km (150 mi) in two days, and then ran back. He then ran the 40 km (25 mi) to the battlefield near Marathon and back to Athens to announce the Greek victory over Persia in the Battle of Marathon (490 BC) with the word νικῶμεν (nikomen[8] ‘We win!’), as stated by Lucian chairete, nikomen (‘hail, we are the winners’)[9] and then collapsed and died.
If I’m reading this correctly, he ran 350 miles in around a week or less? That’s insane.
Humans can actually outrun a horse under certain conditions, notably hot temperatures and extreme distances.