Yeah that "feeling it in their knees" line is way too real. I did warehouse work for a summer and my legs were shot. What finally helped was getting some of those good insoles for my boots, the kind with actual arch support. Made a huge difference by the end of the day. Still had to soak my feet though.
My uncle did carpet for twenty years and his knees are shot now. He always said the padding was the worst part, all that kneeling on concrete. Makes that four mile number sound about right honestly.
Remember my cousin who worked at a shoe store. He said people would come in with totally flat feet after years of jobs like that. Makes you realize how much our jobs can really wear us down.