A reader writes, "For a very healthy and fit person, about how many miles can be walked continuously in 8 hours?" I answered a similar question on how far can a healthy person walk with no training. I have lots of experience with dragging untrained friends and family out to walk a 6 mile/10 kilometer volksmarch. That takes about two hours at an easy pace. They all survived, although many discovered they needed better walking shoes.
From personal experience, exceeding 8-10 miles, even if you are used to walking for 6 miles, is more of a challenge. You can do it, but are likely to develop blisters in new places. Shoes and gear that worked fine for two hours may not be ideal for four hours. This is why you should build up mileage steadily rather than leaping from no walking to walking for four hours straight. In training for a 13 mile half marathon or 26 mile marathon, you walk a long day every week and increase that mileage by a mile a week or two miles every two weeks. There is an amazing training effect that happens. Twelve miles may have seemed very difficult the first time you reached that distance. But six weeks later when you are walking 18 mile days, the first 12 miles are easy and no strain at all.
But how far could a fit, trained person walk in eight hours? I've always finished the Portland Marathon in about seven hours, with no breaks. If the walker is taking breaks and a meal stop, then 20 miles a day is reasonable for a well-trained walker. If they took no breaks and were going fast, they may be able to cover 30 miles.
Is this person going to walk for eight hours day after day, such as on a walk across the continent? Those who have done this have plenty of blister trouble the first couple of weeks, but then either drop out or go on covering 20 miles or more each day. The western pioneers usually covered 20 miles a day with the wagon trains, most of them walking rather than riding. I would definitely class them as trained walkers.