@Lord of Shadows: 'closing at a run' is something I'm sure Archonsod wasn't trying to discount at all. But it is not an extended period of running, is it. The point is, any given person is going to tire more when they are carrying more weight, and having weight on one's limbs makes them tire faster. Obviously people were keen to wear armour almost whenever it was available, but that is because having protection against sharp blades and forceful blows is essential, not because wearing armour a lot makes anyone invulnerable to the limitations it puts on your exertions. People would be potentially able to fight for longer without armour, but glancing blows would be far more likely to cause debilitating injuries and stop you before endurance becomes a factor.
I'm sure soldiers of today get used to wearing their combat harnesses and backpacks, but they still have better endurance without it on. They wear it because they need it to be effective soldiers, not because it is possible to make the weight insignificant to them through training.