In one day? I once walked 30 miles in the Selway-Bitterroot wilderness but I'll never do that again.....unless I have to. 17 miles with a full backpack and I'll never do that again either....unless I get the chance.
It depends on your motivation. If I was being chased by the Blackfeet, a lot. If someone is injured and they are relying on me to save them and get help, a lot. My personal best is 26 miles with a pack. On a good mule 40 miles is not that hard in a day.
Have no idea as to how far I've walked on any given day...However, I've been afield from dawn to near dark (and sometimes after dark) when I used to trap...and then spent evenings fur handling...!! Made for long days...!!
I once waked home from college when stranded without a ride. It was about 30 miles (48km). I was 19 then. The farthest I've hiked in a day now that I'm older was 16 miles (25km), but I find hiking stops being fun after about 8-12 miles (12-19km), depending on the terrain.