I have only owned one pair of running shoes, and don't know how often you should replace them. The ones I have have served me well. They fit perfectly, and I've never had any injuries. I've had them for about 1.5 years, and I estimate I've run about 1600 miles in them. Friends tell me there is no way I should keep running in them and I should replace them immediately, but they feel fine to me. They aren't worn down and still fit as snuggly as they ever did. So, do I need to fork out another £80 immediately, or should I wait until they are visibly worn?