Yoga exercises strengthen your body and make it more flexible. Yoga also calms your mind and gives you energy. In active sports or strenuous exercises, you use up energy. In yoga classes, students report that they feel tranquil after a class, yet have more energy. Slow and steady motion is the key to going into or coming out of the postures. You hold a yoga pose for several seconds or even minutes and give attention to full, quiet breath. Your yoga instructor will always encourage you to relax as the exercises are being done.
Because you gently place your body into each yoga posture the exercises are seen as being low impact and unlikely to cause harm. Done correctly, there's very little chance of injury or muscle stress. A particular asana pose should not be repeated dozens of times, nor are you ever encouraged to push yourself too much.
A yoga session is designed for balance. You stretch to the right and then to the left. You bend back and then forward. You learn to recognize when one side is stronger or more flexible than the other. Thus harmony and balance are achieved with yoga practice.
People of all ages can practice yoga exercises. They are easily modified to meet your needs and physical condition. Don't be put off by the difficult looking postures you may see in a yoga book. A skilled teacher can adapt most asanas by using chairs, cushions, even a wall or other props. A yoga practice can be tailor-made just for you. If something is really impossible for you to do, just forget it. Never compete with yourself or others. Yoga is a stress-free but powerful way to exercise.
Yoga is good for increasing your flexibility and relieving stress, but it doesn't take the place of aerobic exercise. Alongside whatever yoga exercise you undertake you should still do regular aerobic exercise, because these increase your cardiovascular fitness, help you to lose weight, and, for people with non-insulin-dependent (type II) diabetes at least, improves blood glucose control. Yoga can not help with these things.
I am sure that when you start including yoga into your exercise routine that you will feel much better for having done so and the increased energy levels you may well feel will help you with your more strenuous fitness regime - jogging, aerobics, swimming etc.