I used to be in shape back in 2009-2010 and hiked regularly then some sh** happened and well lets see then I did nothing really for like 6 months due to depression and well that saps your strength and willpower to do anything really. Heat also saps my strength as does uncomfortable clothing and armor. I'm 100% serious, the legs they had me wear were like 5 sizes too big for me and hung by a belt rather than a pourpoint. Also I tend to get intimidated when people hit with full force contact, thats not something I am into in all honesty, whenever I duel usually I do light force contact with blunted steel weapons. Yet I did totally fine when I fought bare chested with a sword and shield, didn't tire that much that way. To put it blunt and honestly, heat saps my strength really easily and I dont know why. I did better the second time round though, so I think I may try it again despite myself HATING the armor I had to wear.
I know what you mean, heat is really rough on me too, I'm naturally warm so the heat amps it up even more. I can start sweating before I've even got a full harness on. I do drink like a camel, though.
If the armor isn't comfortable, it's not sized right, or something else is wrong. Is it plate? Post some pictures of it on, and maybe we can give some feedback. I think you should have a go at making a gambeson. In the pics I posted recently with my "full plate" harness, I'm wearing single-layer linen chausses, braies, a t-shirt, and an arming coat - which has no padding, it's just a thick two-layer garment with arming points and lace-tight sleeves. With mail, you'll want more padding than plate since the mail, of course, gives, where plate does not. But you don't need a silly amount, either.
For contact force, if you're sparring / dueling with somebody and they're hitting too hard, tell them to scale back the power. If they won't, don't continue. If they don't respect your wishes for a safe battle, they are not worth the battle.