If you think that you shouldn't set foot in a yoga class because you aren't flexible, then yoga is exactly what you should be doing. Yoga is strengthening and stretching at the same time. Each pose is an isometric exercise, and flexibility is simply one of the many positive by-products of the practice. In other words, you don't have to be flexible before you start practicing yoga. You just need an open mind and a yoga mat.
It may seem at first glance that yoga is about flexibility, because of all the 'show-off' pictures that you see. Yoga is about exploring your limits and not striving to achieve some pretzel-like perfection (even though it looks really cool). However, yoga is really about exploring your personal strengths, limits, imbalances and learning to work with things as they currently exist. The flexibility that occurs as a result of the practice is merely a by-product. As you become more familiar with yoga and the breathing that goes with it, the nervous system will naturally move into a more relaxed state of being. Your brain becomes more balanced as a result. Another by-product of the practice. When this starts to happen, corrosive hormones that are produced by stress will lower and the whole body will feel more energetic and healthy. Not only will you feel better, but you will sleep better too. One of the main causes of restlessness and inability to fall asleep are stress hormones which were very helpful thousands of years ago when we were being chased by animals with large teeth. Turns out these hormones that were intended to help us escape from immediate harm also kick in when we get annoyed with the cost of gas prices, when your toddler poops their underwear in Office Max while you are waiting for your copies, and when you pick up a birthday cake 3 hours before the party and it looks like a 10 year old decorated it. Just practice the poses and breathe...balance your body and mind, and reduce your stress. It's worth every penny and every minute.
Yoga may not be the highest in terms of calorie burning, BUT ~ stretching your muscles as your exert them causes fat to be eliminated around the cells, thus eliminating cellulite. Additionally, as the nervous system moves out of a chronic state of stress the levels of cortisol begin to lower. Cortisol is a known contributor to belly fat. I know dozens and dozens of people that have lost significant amounts of weight (20+) by practicing yoga alone. It has a more of a holistic role in weight loss and fitness.
There are dozens of yoga poses that stimulate various nerve centers, flush toxins from organs, and promote a stable metabolism. For example, twisting poses not only stretch spinal muscles, but they also compress the liver and other organs flushing the tissues out. When the twist is released, fresh blood rushes in. Think of this like cleaning out a sponge. This squeeze and soak process also happens to the thyroid gland during shoulderstand and most twists. The thyroid gland is primarily responsible for producing the hormones that regulate metabolism.
Yoga will help you achieve your fitness goals by building strength, improving flexibility, and balancing the nervous system. Yoga is extremely beneficial for balancing out the right/left, front/back, and top/bottom sides of the body. You'll be quick to notice which side is tighter or weaker right away. Each pose is performed for the same amount of time on each side (we count the breath). These natural imbalances play an important role in contributing to injury and pain. Bring balance to your body and reduce and eliminate pain. Bring strength to the minor muscle groups (as well as major) and you will have balanced strength. Practice regularly and feel and watch your body transform.