I believe the theory goes that you also get more life out of shoes when you give them a break after using them, so 12-16 hours straight for multiple days could definitely have put a dent in that. (Personally, I have way too many running shoes, so they usually get up to a week between uses

.)
Every once in a while I'll try a pair that isn't worth returning but isn't my favorite for running, and I'll retire them early and make them my walking-around shoes. I don't, however, use shoes for running and walking at the same time. They are only walking shoes if they are done being running shoes.
If I didn't have that, I agree with whoever said that it may not be the best idea to use shoes that you retired because they were at the end of their running life - it seems like they could cause the same issues they were when you were running in them. If you do use all your running shoes until they are fully done,
you might want to invest in a comfortable pair of walking shoes separately so you're not hurting yourself.