Shock absorbing capability will diminish gradually and may be inadequate after 350 to 550 miles. The upper of the shoe may not show much wear, but the shock absorption may still be gone. If you are running 20 miles per week, you should be replacing your shoes between 4 and 8 months depending upon your shock absorption needs. It is always cheaper to replace your shoes than to make a visit to the doctors office. Regular stretching may also help reduce injuries.
Runners frequently develop tightness in the posterior muscle groups.
This includes the hamstrings and the calf muscles. The quadriceps and anterior shin muscles may become relatively weak, due to muscular imbalance. The abdominal muscles also tend to be weak on runners who do not exercise them. I recommend stretching one set of leg muscles at a time. One leg is back, knee straight, the other leg is forward with the knee bent.