I don't know about the rest of you, but IRL carrying 50 or 100 or 200 pounds has always fatigued me when I walk around, whether at age 16 or 26 or 36, whether on dry ground or waist-deep snow. Maybe it was different in the Iron Age, who knows?
I can speak to this one. For several years I worked as an elite wildland firefighter, battling forest fires in some of the most remote areas of the world. We were often without support for weeks at a time, so all of our gear had to be carried in with us. A standard day-to-day load would be about 60 pounds, and we would often carry as much as 180 pounds or more.
I think that UnReal World actually does a decent job of modelling heavy pack loads. Once you become accustomed to the weight, it does not necessarily continually fatigue you - however, it does slow and hinder everything that you do. Now, of course, all elite firefighters must conform to a high standard of fitness. Some people might say that it is unrealistic that Iron Age characters would have such high levels of fitness, but actually I think it is the opposite. Our lifestyle as wildland firefighters is actually probably much closer to a prehistoric lifestyle than to a modern man. All day is spent in the woods, tromping around and trying to survive. There is no way to become more fit and mentally tough. I would imagine that even UnReal characters with somewhat low Endurance and Strength are probably still more fit than the average sedentary modern person.
Still, climbing up mountains whilst carrying 100+ pounds is always taxing. A more realistic approach would probably be sort of a "weighted scale" if you will. Weaker characters become more easily fatigued whilst bearing heavy loads, while stronger/tougher characters tire more slowly. All characters fatigue more quickly whilst travelling across hills, and climbing a mountain should tax all but the strongest characters to the limit and take significantly more time than crossing a meadow.