There's an old saying, 'The dosage makes the poison.' Overindulgence in anything can be bad, including exercise. Pushing your body to its limits can make it stronger; pushing your body beyond its limits can cause serious damage. Running and aerobics put an enormous strain on the feet and ankles. Proper shoes can compensate for some of this, but caution is still the best course. If you run, don't increase your distance by more than 10% per week. If you do aerobics, try using a soft mat or trampoline to minimize the shock to your feet. It's especially important to warm up prior to your exercise session. Stretching can loosen up the muscles and tendons of the foot and make them more resilient. Prior to embarking on any exercise program, a thorough checkup by a doctor is highly recommended. For more information on exercise and your feet, consult a foot care specialist in your area.