Using more than one pair of shoes can extend their life and also help prevent injury. If they are different models or types of shoes, the forces on your body are subtly different, which reduces the risk of overuse problems that can come from wearing the same shoes on every run. When you have more than one pair you can stagger when you replace them, which means you are never running in just one worn-out pair and also ensures you don’t suddenly switch to wearing only a new pair. Try to use the shoes to suit your run: you could wear newer pairs on the road runs and the slightly older ones on trails. Don’t push your luck – you could develop an injury from wearing shoes that are worn out.