How far a healthy and fit person could walk continuously in eight hours or the distance you could achieve in a day is a question that can come up when planning an adventure trek. Another situation that could require it is walking unexpectedly due to transportation breakdowns in a natural disaster or personal emergency.

While your body is made for walking, the distance you can achieve at an average walking pace of 3.1 miles per hour depends on whether you have trained for it or not. A trained walker can walk a 26.2-mile marathon in eight hours or less, or walk 20 to 30 miles in a day. Steadily building your mileage with training allows you to walk long distances with less risk of injury.

Even Beginners Easily Survive a 6-Mile Walk in Two Hours

Walkers on the month-long Camino de Santiago trek typically walk 12 to 20 miles per day on terrain that includes many hills.

Source
Very Well Fit