I've heard it argued that the heavily-armored hoplite phalanx of the Greeks wasn't actually invented until well after the Persian wars. If this is the case, what explains the victory of inexperienced Greek militias over the numerically-superiors Persians at Marathon and Plataea?

I've written several answers on this topic before, especially this one and the ones it links to. The short of it is that there were several long-term developments (economic, demographic and military) that made Greece more capable of defending itself than ever before, but the outcome of each battle was still largely down to contingent factors rather than structural advantages.