Walking has the advantage of not requiring any equipment and carrying the least risk of injury of any aerobic exercise. Use good posture, swing your arms, and keep a good pace. Three miles should take about 45 minutes. Specific forms of exercise have their own benefits. Swimming is great for the joints, a balanced muscular workout, and relaxation.
Cycling builds knee muscles and can provide a feeling of exhilaration.
Dancing is one of the best aerobic activities of all, because it's fun, never boring, and provides a thorough workout. Once you have developed good habits of regular exercise, begin to add stretching, muscle toning, and strengthening to your routine. Yoga is a great way to stretch, improve flexibility, and experience deep relaxation. Breathing exercises, meditation, and other forms of relaxation are important to help neutralize stress. Personally, I walk whenever possible.