Running in high mileage, old or worn-out shoes is one of the most common causes of running injuries. As you put more miles on your running shoes they lose shock absorption, cushioning and stability over time. Continuing to run in worn-out running shoes increases the stress and impact on your legs and joints, which can lead to overuse injuries. When walking your feet absorb 1-3x your body weight, running 3-5 times your body and jumping 5-7 times your body weight. This means that depending on the intensity of your activity, you will also wear down the support levels of your shoes faster.
So how do you know when shoes need to be thrown out? Don't use the treads of your running shoes to determine whether you should replace your shoes. The midsole, which provides the cushioning and stability, usually breaks down before the bottom shows major signs of wear. If you've been feeling muscle fatigue, shin splints, or some pain in your joints -- especially your knees -- you may be wearing running shoes that no longer have adequate cushioning.
A good rule of thumb is to replace your running shoes every 300 to 400 miles, depending on your running style, body weight, and the surface on which you run. Smaller runners can get new running shoes at the upper end of the recommendation, while heavier runners should consider replacement shoes closer to the 300 mile mark. If you run on rough roads, you'll need to replace your running shoes sooner than if you primarily run on a treadmill. Mark your calendar when you buy a new pair of running shoes so you remember when to replace them. If you use a training log, be sure to record when you bought new shoes -- it will help you track how many miles you've run in them. Writing the purchase date on the inside of each shoe's tongue is another good way to help remember when you first started running in them.
About halfway through the life of your running shoes, you might want to buy another pair of running shoes to rotate into your runs. Your shoes will last longer when you allow them to decompress and dry out between workouts. Also, having a fresh pair of shoes as a reference will help you notice when your old ones are ready to be replaced.