Only you can determine the best practice and choice for you.
However, the Ashtanga series is a fairly rapid sequence of movements, some of which require both mobility and stability. I personally would not direct a beginner who did not already have a background in physical activity AND heightened body awareness to this particular practice. Add to the mix that there is back pain present and it may be ill-advised. But again, each must determine this for themselves in their own body and their own experiences.
Perhaps begin with a gentler practice that is alignment-oriented. The ones I think fit this bill are Purna Yoga™, Viniyoga, and Iyengr yoga.
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