Running in old or worn-out shoes is a common cause of running injuries. Your running shoes 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.
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 keep track of when you will need to replace them.