At an average walking speed of 3 mph, an able-bodied man or woman can cover 72 miles in 24 hours of uninterrupted walking. The distance actually covered would likely be somewhat less than that, as few humans are able to walk at full speed for an entire day.
The distance covered in a day's walk can be affected by the terrain. Walking uphill requires more energy, and walkers often move slower when walking up a hill and faster walking downhill. Rest periods would further reduce the distance covered in a day, though some time could be made up if the individual alternated walking and running.