Hi Chris. When you say worn down, can you be more specific?
Is it the heels, the outer sole, under the big toe, or all over...?
Have a look here...
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Bit scary! 300 miles!!! But I think they are talking about road running.
Some shoes can be resoled. Not cheap but can increase the length of shoe life. Little point in resoling a shoe where the mid sole has degenerated, or where the uppers are past their best.
I use Saloman running shoes, which are not cheap. They seem to last me a fair time, and I find them comfortable.
Good hunting.