Who said anything about marathons?
You mean a tactic designed specifically to defeat mounted knights in full armor was effective against mounted knights in full armor? Astounding.
Just to beat the dead pony into a bloody pulp:
The weight of the full plate armor figures prominently in it's history. Though the fellow in the video sprinted 50 yards on a level road, the mother nature was rarely that kind to the full plate armor. The real battle would start lining up just out of arrow range about 300 yards away, and defenders would be (if they are not complete morans) up the hill. And if you ever ran across a fallow field or in snow or in mud, you would know how hard it is compare to a level park road.
Further, history is full of examples where a dismounted knight in the full armor died due to the weight, bulk, and poor thermal management of their armor:
Agincourt, 3rd Battle of Jerusalem, Ladoga Lake battle (accidently mounted), Grunwald forest battle...knights were dying in the mud, sun (due to dehydration and heat stroke), sinking under the ice of the lake, or plain heat exhaustion.
Further, their capacity to charge was very limited by the strength of their horses, even much lighter cuirassiers of Murat bungled their charges because they would start galloping 600 yards away from enemy lines, and by the time they would reach the adversary their horses would be so exhausted that they would fall or walk.
So "marathons" need not be in a definition, simply covering 300 yards and than fighting would take thew wind out of a worrior in a full plate, due to simple terrane.
Further, the pole armed infantry formation was not a tactic specifically designed to defeat fully armored knight, it was a universal approach (until and including tercios tactics) to defeat any enemy.
Finally, what early musket would not do, all sorts of pole arms would-penetrate armor. In fact the best cavalry of the 14-15 century-Hungarian mercinaries barely wore any armor, and they were trashing europien heavy cavalries with a war hammer and speed.
Even in moder times tanks had to be supported by infantry because defending infantry could take out unprotected tank with a grenade bundle or an RPG.
In Fallout PA absorbed the best qualities of the armor without giving it all the drawbacks, which is not kosher in my book.