Most of the narrowing in the arteries and viens occurs between the knees and the ankles resulting in markedly decreased circulation to the feet, hence the pain in the legs or feet when walking.
When walking the muscles require more blood supply to bring more oxygen to the muscles and remove toxins.
If the arteries are diseased the muscles do not get the extra blood they require and so toxins build up in the muscles causing them to ache and become weak.
When the arteries are severely narrowed or blocked, even at rest they cannot supply enough blood to the legs.
The part of the body furthest away from the heart is affected first, namely the toes and feet.