Yoga can be adapted to bring benefits to any body. When I first began my practice, every part of me had something to say after each yoga class! I discovered spaces and muscles in them between my ribs that I had just never known before. My legs would shake in standing poses, and my breathing would be slower than that which the teacher was directing. My shoulders were chronically tight. Gradually my body began to find its way into the patterns of breath and movements. After 8 years of experience, my yoga practice always begins with awakening of this deep coordination of breath and muscular activity. My hips stiffen every night, my shoulders need to respond to an exploration of their range of motion before I ask any thing else of them. Though my legs have flexibility, the big hamstrings need to warm up before going for a full forward bend.
The first aspect of a safe and healthy practice is to let the judgments of yourself go. In order to listen to my own body and use it fully as the vehicle for my practice, I have to be willing to notice its actual condition without the overlay of pre-set ideas about myself. My students quickly discover that it is their own knee and hamstring combination that will tell them how long a stride to take in their warrior pose (Virabhadrasana I). Whether long or short, it is the knee over the ankle that will protect their joints, softening the shoulders, finding the breath deeply moving through, and this is what will allow them to continue to explore the openness in their hips, and the ease of their spine. It is through this process of finding their foundation that begins to release into the support that allows longer holding of the asana and movement within it. Overriding their own needs, and over-reaching cuts off all the possibilities. As the body opens, so does the stride. Taking a posture to look a certain way without listening to your own body is not yoga.
So it's great to take a beginning class, a slow flow class or a more technical take-the-asana-apart workshop in order to work with a teacher who can offer the adaptations and reasons why using a block or a blanket will help you gain the benefits of a posture. Teachers usually explore a series of postures and preparatory actions that help open your body, your breath and your mind for the more advanced asana to come. At each step along the way, noticing what you can relax and release, noticing the source of your foundational support, and keeping your attention on the breath will help bring you deeper and deeper into your practice, no matter what "shape" you are in.