Walking on the Treadmill is the same as walking on the tracks of the park outside. Treadmill has the advantage that you can do your exercises inside and are independent of the weather conditions. Only, check that the surface is soft.
You have the advantage that if you want to do walking against resistance, on an inclined path, you can do the on the treadmill. When walking on a slant, maintaining good posture with only a slightly bend at the waist is best for your back.