Most people walk at a speed of 3 MPH so if you walk at a speed of 3 MPH then in 1 hour you'd walk a total of 3 miles.
In 20 minutes you would walk a distance of 1 mile so if you walk 30 minutes you would walk a distance of 1 1/2 miles and if you speed up your walking pace you could cover 2 miles in 30 minutes.
But walking 3 MPH is a good walking speed and covering 1 1/2 miles in 30 minutes is a good pace for walking.