While it might not be 100% guaranteed that the body will go where the mind leads, the message is that you can use the mind to guide and heal the body, to overcome physical obstacles, and to accept the ones that can't be corrected.
There are two ways to sustain a healthy body and mind: inner work, and working from the outside with medication, diet, and physical exercise. Inner work and treatment from the outside complement one another. One without the other is far less effective.