If you are looking for a simple aerobic workout -- running may be the best choice. It requires no equipment and the intensity can be controlled by your pace. If you haven't been active, take it slow. Go for duration, not speed. Feel free to combine walking and running. Make sure you warm up by walking at a brisk pace for 5 to 10 minutes. Stretch after you run and not before. During the summer months stay hydrated by consuming at least 1 liter of fluid.