I remember in the late 80's walking into a martial arts class, where this fat bald old guy was teaching a class of about 25 black belts, including state and national champions up to about 5th dan.
I bet he could not do half the stuff he was teaching, but fuck me did he have some kick arse students, I was awestruck just watching
So someone does not need to look like a buff body builder or fitness model to know what they are doing and may be be a great teacher/instructor/coach.