It is harder to walk uphill than downhill because you must lift the weight of your body and to do this requires greater energy than that needed for walking on the level. To create this greater energy your muscles require to give extra lift, your heart has more work to do to feed the blood cells and remove their waste matter and your lungs have more work to do to remove the carbon dioxide from your heart and replenish it with oxygen. That is why the steeper the climb and the more concentrated the effort, the more quickly you breathe. If you are out of condition you start to “pant” to glup in extra oxygen. In comparison to the energy necessary for walking on a horizontal place, the total value of the extra energy needed for climbing is the weight of your body times the height you are to reach.
The steeper the incline of the hill, the quicker you use this extra energy. It is therefore harder for you to walk up a steep hill than a gentle one, although the energy used up in either case, where the height to be reached is the same, is identical. When you walk downhill, very little energy is needed because the weight of your body carries you down the slope. Of course, this is the pull of gravity that is helping you down, in the same way that you have to overcome the force of gravity when you walk up a steep hill.