People on long trails either get into fantastic shape or start to break down somehow. The ones who get into super shape regularly do multiple 26 mile days on the AT. I've hiked with several before dropping behind after a few days. Not to start a controversy, but folks who have done the triple crown say the AT has much harder walking than the others, so I suspect higher mileages are practical on other trails.

My personal experience is that on the AT 70-year-old men and women will do 26 miles now and then and some 60+-year-old men and women do 20 miles almost every single day. Occasionally, someone will to something completely nutsoid such as crossing Maryland in one stretch or 50-60 miles in a day just to have bragging rights.

It is also true that folks who exceed their recovery limits don't do well the next day. But again, long trails can turn people into motor monsters.