Yoga is an ancient Indian science, which guides us through a series of mind/body self-purifying practices, with the intention of reaching the final goal - Samadhi, spiritual liberation, self-realization, enlightenment. The physical practice on the mat is just one of the "divisions", as we can read in the ancient Yoga Sutras of Patanjali. We start with the body, learn how to use the breath as our weapon against the ego, and only then move on to the mind, through the more complex and demanding practices of meditation and concentration. However, we won't be able to sit in stillness and do the required work if our joints ache and muscles are tight.
So, while the toned and flexible body may (or may not) become a byproduct of the asana practice, contrary to the popular modern belief - it definitely isn't its goal. Yet we do need to start on the mat, with the physical, to prepare it for the more serious work which is meant to follow. Asana's goal is to do just that - to get our most gross layer ready so that we can sit, breathe, go deep, transcend.
I have always known and understood this on the intellectual level, but my Vipassana retreat has proved it to be SO true experientally. I attended my first Buddhist meditation retreat about 6 years ago in England, and even though it was just a 3-day course, and nowhere near as intense as Vipassana, I still recall how much I suffered. Despite having always been naturally flexible, I wasn't able to sit still for more than a few minutes without getting pain or numbness. I had to constantly shift and change, achieving very little depth in my meditation, and at the end of it felt physically broken. My lower back in bits, hips sore, neck stiff.
Fast forward to now, with a good few years of daily practice behind me, and the last months of almost daily Yin Yoga. I've just spent 10-days sitting down for up to 11 hours per day. No big deal. From the Day 4 onwards, I've discovered that I am able to sit still straight for an hour, not even making the slightest movement. And aside of a random muscle spasm near my right shoulder blade, which disappeared the moment we went deeper into the technique, I've had ZERO problems on the physical level. No aching hips, no sore knees, no issues in the lower back. Of course such intensity of sitting down does take its toll on the physical level, so I felt that the body was perhaps a little stiffer and dull, but I was fine. I was fine to sit, to breathe, to go deeper. The body wasn't blocking the process of meditation, it was supporting it. It was holding the space for me to do the work. Thanks to which, I believe, I was able to experience deep transcendental states of consciousness, having gone beyond the physical. WOW!
Patanjali knew what he was talking about, when he wrote the Yoga Sutras. He put the eight limbs of Raja Yoga into a very specific order for a reason. Very few of us, Westerners, have bodies which allow us to sit on the floor for hours on end. We need to open those hips, lubricate the knee joints. We need to strengthen our abdominal muscles to support the spine. It does start on the mat. But don't let it end there!