I read a rather interesting article in a historical journal a while ago talking about what kind of shape a "knight" from the 15th century would be in and what the physical results of wearing armour would be. The historian's primary focus was looking at what kind of body type fit into various suits of armour but also looked at the skeletal structures of people who were known to be trained fighters. His conclusion was that your average professional warrior was between 5' and 5'6 (so reasonably tall for the time) and weighed about 160-180 lbs. In other words, tall and lean. Very little of this weight would have been fat and if the armour was custom fitted then it would be remarkably easy to move around in. The mechanics of it weren't unlike modern hiking backpacks. The bulk of the weight was carried on the hips, leaving the upper body relatively unencumbered. If the armour was properly fitted and worn then the person could move with considerable freedom.
The second part of the study was even more interesting when he examined the effects of wearing full armour. He found the biggest sticking point is endurance. With a visor down, ventilation and breathing become a huge issue. He had volunteers run on a treadmill at a fairly leisurely pace (I can’t remember exactly what speed it was set to) in replica armour without visors, with "round armour" style visors, and with "gothic" style visors for set periods of time and measured their bodies lung capacity, blood oxygen saturation, heart rate, and all that jazz before and after 5, 10, 15 and 20 minute runs. His conclusions were about what you would expect. After 5 minutes everyone could still perform admirably but after 10 minutes the performance of anyone with a visor began to degrade considerably. The difference was more pronounced with the "round armour" than the "gothic" though only one individual wearing a visor was actually able to complete the 20 minute run.
In other words, you can do just about anything in plate armour, it's just a question of how long you can keep it up. It's more than likely that during long battles soldiers would have come up with a way to rotate in and out of the ranks since even a few minutes resting with your visor up can be enough to catch your breath. It also shows just how important a knights "shock" value was. Once a fully armoured man was committed to a fight it had to be over within a few minutes or his performance would degrade sharply. That's a big reason why so many knights relied so much on their horses and the ability to charge, since an effective charge could quickly scatter the more poorly trained infantry and not cause the cavalry to become bogged down in a melee.
It's a pitty that some sort of endurance rules couldn't be introduced into D&D but I think that any attempt would be seriously overcomplicated and likely just end up bogging things down.