When I was a laborer at a landscaping place, it still took me many weeks to work up to covering 40 miles in two days (flat, open terrain) with a 50 lb load.. I started with 30 lbs, and 15 miles a day kicked my butt. You have to harden your feet, or they'll blister, even with proper socks and shoes. A great many of those who "think" that they can "run and gun" with 40 lbs of gear are full of it.