Heroic sacrifice is a very common trope in propaganda.....Also, I wouldn't claim him to be the overall hero of the battle of marathon. That would prob be Miltiades, Themistocles, or Aristides.

The popularization of the tale of someone running matarathon wasn't popularized until Plutarch wrote about it nearly 500 years later and he named the runner "Eucles, although also possible Thersippus of Eroeada", then around 100 years later Lucian of Samosata gave the same story but names the runner Philippides (not Pheidippides).

Philip the demigod was so tuff he could run for two days straight and not break a sweat.

And there is a difference between mythology and history.