Look at modern sport fighters, like boxers - they are in excellent shape and train for endurance, yet they are noticibly tired after a few minutes and exhausted if the fight lasts any length of time. And, they aren't wearing armor or swinging a weapon.

I saw an episode of Mail Call on the History channel where R Lee did some sparring in full plate. I believe it was about 90lbs of gear. Just wearing the armor, he said it didn't feel that heavy because if the way it was distributed over the body, but when he sparred with another guy they were both out of breath very quickly. Once they were both too tired to dance around and do anything fancy it basically turned into a smashing contest on each others shields and the winner was who had more energy, and could continuously deliver crushing blows to the other guys shield and keep him off balance until he fell down. Granted neither of the men were in great shape or great fighters, but it still gives an idea.