Hmm, I go backpacking often on six day trips into wilderness, often off trail at 12K. We're beat by 10mi and start slogging past 13mi. And we dialed our gear down to ultralight (15lb packs with canisters). I think you may be projecting your own fitness level onto everyone else. 18 miles over 2 days is perfectly reasonable for me if you're not going to the gym 3-4x a week and training for 10Ks and marathons.
In the UK older high school kids will go on Duke of Edinburgh expeditions where Gold is the highest level, and they are going 14 miles a day carrying packs. That is young, healthy 17-year-olds. I deliberately didn't do it because I wasn't fit enough.
I'm only 38 now but I wouldn't keep up with those youngsters. 10 miles is a good day's walk for a normal person, or 6 miles for a pleasure walk.
