There is a massive misconception in the mainstream that yoga is
just about stretching. The practice of Yoga is far more than just
stretching, it helps to develop and increase your respiratory system.
The respiratory system is the organs that supply the oxygen to your
body. i.e. Your heart and your lungs.
The breathing techniques that are vital in this practice will aid and assist in learning how to use your heart and lungs effectively and efficiently. You don't have to hammer your body into the ground as you would by running. Therefore they will get stronger and better at their job without having to be hammered in doing so.
With the power of focus and control you can have the ability to keep your heart beat at a steady rate and use each breath to it's fullest potential without the huffing and puffing that you can see going on in other forms of exercise.
Huffing and puffing is a form of having a lack of mental control with the body. That is why yoga is so great. To make the gains in yoga one must maintain their control and if you are struggling with your breath, your body will be all over the place. As you know yoga means "to union." This is the union of mind and body.
Please don't get us wrong, running and aerobic classes are good but good is not enough. You must strive for excellence if you are to grow, improve and be the best you can be. What's the point in looking to get fitter when you are smashing your knees, ankles and other joints into the ground while you're doing it. Not to mention the tightening of your muscles as well.
You don't have stop running and aerobics if you don't want to, however you can minimize the damage and side effects that comes with them by incorporating yoga into your fitness regime.