To achieve real success, the school owner or martial arts teacher needs to establish a "practice of training" that gets and keeps them in peak shape.
But recognize that "peak shape" for the average person is much different than "peak shape" for the martial arts Master-Teacher.
Our peak shape doesn't rely only on the flat stomach; it requires that we master martial arts skills, to the best of our abilities. Granted, there are coaches and teachers who aren't absolute physical masters of the "art" they teach, one that comes to mind is the world-renowned gymnastics coach Bela Karolyi. You won't see Mr. Karolyi doing flips on the balance beam, but he is still one heck of a coach. Of course, he is not obese either.
Nevertheless, the martial arts has a tradition of older masters being very proficient, age considered, in their arts. Morihei Ueshiba, Jhoon Rhee, Ernie Reyes, Sr., Helio Gracie, Wally Jay, Dan Inosanto, Hidetaka Nakayama, Joe Lewis, and Bill Wallace are fine example of that tradition. My opinion is that your own fitness and skills give you credibility that instructors who no longer walk the talk of their work don't have.
In my training program for martial arts teachers, The Ultimate Black Belt Test (UBBT), I don't tell participants what art to practice; I just encourage them to practice. One of the most important factors in practice is to have something to practice for. In the UBBT we have three meetings over the course of a year, with the final one being the graduation test. We also require members to give 10 public performances during the year of their training; all good reasons to train hard and to be fit.
You don't really have "success" if you don't enjoy vibrant health and if you aren't using the practice of the martial arts to achieve the very concepts you try to promote as a benefit of practice. You look and, most likely feel, like a fake.
As a consultant to school owners, the first place I look when trying to help a teacher is at his or her USE of the martial arts. If a school owner isn't using the martial arts in the very way he or she tries to get students to use it, then the apple that is the school and business is wax; it looks like an apple from a distance, but upon closer inspection it reveals itself as an impostor.
Rule No. 1 for a martial arts school owner, don't sell what you don't live.