Running shoes are typically only designed to last for at most 500 miles. It's just hard to make a foam material that can provide cushion but also not break down over time (and still not cost a fortune).

I think the best practice is to only use your running shoes when you run. Wearing them around town as you do errands just adds the amount of time you are compressing the foam in the soles. That said, the old running shoes still work fine for walking around after the 500 mile mark. They just don't provide much cushion to absorb impact.