Carrying stuff is tough. One of my favorite army stories...which I'm sure I've told here before...is moving through the thick bush while on field exercises and every branch and limb jammed into my web gear, my helmet, my helmet strap, my M16 sling, etc etc. It made the march extraordinary interesting (I used different words at the time).
But in games PCs tend to rack and stack more gear on their backs than a dog has fleas. Throw in some heavy armor, four or five weapons (or heavy books) and you have a recipe for a 1-2 mile hike per day with exhaustion being the rule.
Of course the modern trooper carries a mountain of gear into the field and manages to do it with some success, and the Romans did as well, so I suppose with training, practice and care in the packing it is possible.
It is the Adventurer's Backpack.
Here's a Roman Light.