I've done a fair bit of weapons fighting and training (20+ years in various asian martial arts, including Kendo/Kenjitsu). I've worn what would be equivalent to light (stiff leather) armor in kendo, but that was limited to the torso and the helmet. I also wear leather for my costume at the local renfair, and even in temperatures of ~100F, it's not that bad compared to just wearing appropriate attire anyway.
I think Christopher Taylor's ideas are probably the best for characters/adventures that are mostly situated in or around a city. A warrior might only put on his full plate when going into an actual battle (Think of the different armor that Faramir wears in the Lord of the Rings for when he's 'scouting', and when he lead the charge to Osgiliath). Even heavy armor isn't really much more exhausting than medium or light armor.
The things I would maybe impose is a max run speed. Even this is somewhat artificial, though. The other things encumbering a typical adventurer are running with a sword in a sheath, the backpack, cloak, etc. (Running with a sheathed sword can be pretty comical actually, and it's very encumbering in terms of slowing one down.)