http://en.wikipedia.org/wiki/World%27s_Strongest_ManFind "farmer's walk" on page. Then see
http://en.wikipedia.org/wiki/Lifting_stone and
http://en.wikipedia.org/wiki/Husafell_StoneHere we have folks lifting fairly unwieldy loads (not strapped to the body and well-distributed, or in backpack, etc) of 400+ lb for a hundred feet or so in the latter case, and 300+ lb per hand in the former.
http://thedonovan.com/archives/modernwarriorload/ModernWarriorsCombatLoadReport.pdf pg 8, section 8.4: modern US army emergency march load per soldier is 120 to 150 lb for several days at 20km per day. (Mixing measurement systems!) That's an expectation of the average soldier, not just the beefy hardasses.
In Randolph B. Marcy's
The Prairie Traveler, 1859, "The Spanish Americans are, however, cruel masters, having no mercy upon their beasts, and it is no uncommon thing for them to load their mules with the enormous burden of three or four hundred pounds." (p.100), and "One hundred and twenty-five pounds is a sufficient load for a mule upon a long journey." (p.109).
In John D. Billing's
Hard Tack and Coffee, 1888, Civil War soldiers are described as being issued or trying to carry large amounts of gear but generally throwing most of it away along the roadside because it was heavy and/or bulky, to the extent of keeping only his daily rations (some 2 pounds), bowl and cup, basic clothes and shoes (one piece each), and of his overcoat and wool blanket and rubber blanket keeping two, and possibly his half-tent (which he was supposed to set up with another man to make a whole) and of course his gun / bullets / powder, and whatever small incidentals were in his pockets.
Medieval re-enactment people (SCA is all I know of firsthand) typically run around fighting with 10 lb clothes and boots, 50-70 lb armor, 8-10 lb shield, 6-8 lb weapon (total 70-100 lb) with greatly slowed walk and run speeds and in great danger of heatstroke depending on the climate. Ten minutes' fighting, six times over a few hours, plus movement between, is exhausting.
I'm an athletic-built person but honestly I'd describe my lifestyle as sedentary (I'm not out hauling bales of hay and throwing pigs and riding horses and moving 200 lb stones out of my field). I can lift and carry a 250 lb person a short distance over smooth dry cement but it feels like if I step wrong I'll break my ankles and knees. I can push around a wheelbarrow (the kind you lift two handles and it rests on one wheel in front) carrying six cubic feet of gravel (600 lb according to
this) all day long.
Wikipedia says the energy required to pull weight supported by a 4-wheeled device rather than dragging it on the ground would be 1/40th, suggesting that if you can pull a 4,000 lb wagon at maximum you are able to drag only 100 lb on the ground at maximum. I know that I can push a fuel-less VW New Beetle (3,000 lb total weight estimate for vehicle plus occupant and cargo) over flat ground with some difficulty ... in flip-flops, which really didn't help. Let's say the friction problems associated with dragging versus carrying mean I could drag a 75 lb load such as a log under the same circumstances. With proper shoes and harness ... well, let's leave that experiment for Saturday. Yet this is counter-intuitive, since I know I can shove a heavy thing that I can't lift. Luckily this is just a side point on the topic of dragging vs. pulling and might have little bearing on carrying.
In summary, some aboriginal Finnish bruiser carrying 300 lb isn't impossible ... but carrying that load through a trackless forest over a long distance is no mean feat, and dragging it harder still.