Ever tried fighting for hours with a sword? It is no less exhaustive in my experience and as you know I have experience with long spears as well as swords- in fact sword is if anything more exhausting.
There is probably little difference in the rigour of the phalangite or legionaire training. In fact I imagine it to be very similar as they were both elite levies. But the emphasis I would think is different, the legionaire would train mostly with Pilae and Gladius Hispanensis with some unarmed probably added whereas the Phalangite would train Sarrisae (sp?) mostly and some sword and pancration. However, the Phalangite sword was not as effective as the GH and the legionaire would be better at it cause his emphasis was on it, while Phalangite was on sarrisae. Just like an artellerist or tanker today has had basic infantry training, but are no match for a trained infantry soldier and the latter cannot really drive a tank. The comparison is not totally analogous, but I think you know what I mean.
Thus when the character of an engagement would change from the Phalangite's terms to the legionaire's, the latter would of course mostly prevail. So when the Phalanx lost cohesion and it went to sword distance instead, the Romans had an advantage.
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