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. Not only will you feel better, but you will sleep better too.
Don’t let a stiff body be an excuse never to start yoga. You should just start practicing from wherever you are right now. Don’t judge your practice by how far you can or can’t stretch. Never feel inadequate because you can’t hold a pose for long, or because it doesn’t perfectly mirror the pose in a picture. Practice spreading your awareness through your whole body.
Just practice the poses and breathe...balance your body and mind, and reduce your stress. It's worth every penny and every minute.
Just start the journey. You never know where it might take you.