www.nps.gov/yell has a map with distances. The Grand Loop around the park is 140 miles long. That's a lot of walking. There is no bus service for getting around. That hiking will not be flat land hiking. There are mountains to climb. It is also at elevation, 7500 feet to 8500 feet which will be a factor if you aren't acclimatized. In my estimation, Yellowstone is not a park that can be explored only on foot. One more thing, if you leave awning out while you are gone,, there will be about a 90% chance it will be gone when you get back. Big winds come up very quickly, taking anything that isn't nailed down.