I am going to say this on record - nothing beats going to an actual Pilates mat class. An hour long class works every bit of your body, while stretching and lengthening you as well. Plus, the instructor is right there providing feedback and giving corrections, as well as discussing the subtleties of the Pilates method.
Now if you can't make it to a class, there are DVDs, but good Pilates DVDs are hard to find. The Winsor Pilates DVDs are broken up into 20 minute targeted workouts, and The Back Workout is no exception. However, this DVD is different since it was created to alleviate back tightness and pain. Winsor even warns at the beginning of the workout that if you suffer from chronic back pain, you should consult with your physician first. This DVD is definitely meant to help folks with tight weak backs get stronger so they can take regular Pilates classes.
The series begins with stretches for hip and thigh muscles and a spine warm up. Classic Pilates exercises are taught in very modified variations that are perfect for beginners since they are accompanied by considerably more details than many of the workout videos by Winsor. However, if you are not dealing with a bad back, this is less of a workout and more like a warm up or cool down with gentle ab work. I feel following this DVD would be a great way to prepare for a long run, or help you chill out at the end of a long day. If you do have back problems this series could be very beneficial for you, but like Mari said, you should consult with your physician or physical therapist before trying the DVD.
The Back Workout DVD is included in the Best Value package (which also includes 8 DVDs, a sculpting circle, exercise planner and cookbook) on the Winsor Pilates website for $79.80. You can also find individual copies for as low as $7.50 on Amazon.