The average person can walk about 3 miles/hour on flat, easy ground. Therefore if you can keep that pace up for 12 hours, you could potentially walk 36 miles and if you keep that pace up for 24 hours you can walk 72 miles a day
The average human being can walk approximately 3 miles per hour. If this number was then extrapolated to the period of a day (12 hours) this would equal out to approximately 36 miles walked.