I've done retail and it's physically taxing especially if you are standing all day. First of all, you MUST have comfortable shoes. If you don't then you MUST invest in a good pair of shoes. Secondly, when you get home, it sounds like stretching or a warm soak would be a good idea. Eating unhealthy food and gaining weight will only make you feel worse. The additional weight will put more strain on your legs and back.

Realistically speaking, retail may not be for you. There are places were you can take courses in office skills (typing, computers, etc.) for desk jobs. I suggest you consider doing that. It sounds like your body cannot stand all day. It's nothing to be ashamed of. Consider making the change.