Most people can carry 40 pounds a mile or two without great difficulty. With a decent backpack.
40 pounds is quite a bit of food.
Might have some trouble with frozen stuff melting before you get it home.
You're talking about a serious, "go-camping-in-the-wilderness" backpack...not a typical size one that a student might use for carrying books, or a cyclist might have.
Even if I agreed with your proposition that "most people" could carry 40 lbs. The idea is preposterous because it's impractical to do this on a routine basis.
I live in a suburban locale, about 15 minutes from a mid-western city. It's by no means remote. However, it would be a two-mile walk one-way to the grocery store. Sidewalks would be pretty much non-existent outside of my neighborhood, and I'd have to cross 4 lanes of traffic where no crosswalks were anticipated.
But let's say I ignored the safety issues present, and walked anyway. I'd still be faced with a minimum of a 30-minute walk to the store, and that would be hoofing it. 30 minutes to walk to the store, another 45-60 minutes to shop (average time), and probably 45 minutes back to my house. If I planned correctly, I could do this on the weekend, but that's 2.5-3 hours wasted.
If we got to some post-apocalyptic version of our country and I had to do this to survive, I would. But now, why? We live in a country where we shouldn't have to walk 5 miles to the "community well" for fresh water. It's called civilization, which should be the by-product of our hard work, productivity, and ingenuity.
LOL. Have you seen the physical shape of the average American? Granted it's easier to carry 40 lbs in a backback but the average American could not do it at that.
Heck the average American can't walk a mile without a backpack.