"Or by your average bum ass hipster. Romans did most of their campaigning in the summer, so figure, 10-12 hours of marching at 15-20 minute miles (3-4 mph), which is a pretty standard walking pace. I've made 47 miles in one day on the AT, and that's over rough terrain from a guy who isn't in fantastic shape. It's also worth mentioning that a 40 mile/day march for a field army is a "forced march," and Roman armies actually rarely made more than 12-15 miles a day under normal conditions. As a practical matter, the real limitation for a forced march is not the limits of human endurance, but the limits of the army's animal transport and cavalry element. A young adult male in good physical condition can easily maintain a 3-4 mph pace for 18 or 20 hours - if they absolutely had to - over even ground. A horse - especially an unshod horse (like the Romans used) - can't give you much more than 10-12 hours at that kind of pace."
Satan is my Stewardess;
I agree. Did 10 miles a day this summer with 60lbs on my back (I weigh about 135) at 9000 feet (I live at sea level) and I am a smoker. In 1914 the German armies in France marched 30+ miles a day for almost two months... and engaged and defeated larger French forces both during the advance and at the end of the advance. A British SAS squad sent behind Iraqi lines in Desert Storm ran 150 kilometers in two nights (hiding from the Iraqis during the day) with 200 pounds of shit on their backs... well, the dumped their main packs after the first night and only carried ammo and water but two gallons of water and 1000 rounds of ammo are still going to weigh about 50 pounds. In our current war in Afghanistan US Delta operators hiked 20 miles at night, at 10,000+ feet elevation, in chest deep snow carrying 150 pounds of shit on their backs to get into position to call in air strikes on Taliban positions. Many more modern examples abound, try reading up on British marching exploits in the Falklands War, or about the Mujahideen in Afghanistan (current war or the Soviet war, although this is probably a bad example as Afghanistan is still largely a primitive society in the physical sense). Many German units on the Ostfront in WW2 marched 40+ miles a day (either advancing at the beginning or retreating at the end) with full kit while engaging (and often defeating) vastly superior Soviet forces (10-1 by the end of the war) and many with chronic dysentery, malnourishment, wounds, jaundice, etc.
The point is, I think that among our best the potential for great strength and endurance is still there, it just lies more or less dormant, and because of herd politics and 'rising living standards' the overall mean fitness and potential fitness have probably gone down.