The body, too, should stay firmly composed, and not fling itself about either in motion or at rest. Just as the mind displays qualities in the face, keeping it intelligent and attractive, something similar should be required of the whole body. But all this should be secured without making an obvious point of it.
The art of living is more like wrestling than dancing, in that it stands ready for what comes and is not thrown by the unforeseen.
In the first part, I believe Marcus is saying nothing other than, "keep your body physically fit; and don't be pretentious about it." The point being the body and health of the body should be treated as indifferent. The point and purpose of life is not to be the most fit. For some people, all they can think about and do is keep their body fit and in shape all the time. Life is about the mind and virtue, not about having the most fit body. What good can come from having a perfect body, but no virtuous mind to go with it? So, take care of your body, but don't do so at the expense of caring for your mind.
The art of living: be ready at any moment for any event.