As someone who has lost 70lbs, become both a group fitness instructor and a personal trainer, and gained a few of those pounds back, I also take the contrary view.
The super lean, ultra fit physique is very hard to maintain if you're trying to have a healthy, balanced life. I sustained a disk injury in my lower back a few years ago because my expectation to work full tilt with my classes 12 hours a week, demonstrate exercises with my personal training clients, and work to foster my own physical goals was unsustainable. I was forced to take everything down a huge notch and lower my expectations of my physique, for the sake of both my physical health and my other-life balance. Do I know what I'm talking about when I tell other people how to lose weight, become healthier, and look better naked? Absolutely! 100%! Am I willing to make the sacrifices and lifestyle changes necessary to get that way again myself? Absolutely not! But I still know how to put together a tough workout, how to cue it extremely well, and how to motivate every person in that room to challenge themselves to their own personal limit.
Why do we put such high expectations on instructors and personal trainers? How many glorified and haloed coaches do you see in mens' professional sports who look more like an overweight couch potato than an athlete? It's a nasty double standard.
Eating disorders and body image issues are rampant amongst female physique competitors, trainers, and instructors. The image of the "healthy" female is to have a level of body fat lower than most women can reasonably sustain. Keli Roberts was a keynote speaker at a fitness retreat I attended two years ago, and has a personal journey that moved me to tears. Her story is one of extreme pressure to look super thin (she began professional modeling in Europe at a very young age), which led to a life of heavy drug use and mental illness. She eventually recovered from these challenges after finding her way into the fitness industry. She speaks very powerfully about the poor body images of many, many women in the fitness industry and coping with those pressures.
Just over a year ago, primarily because of my degenerating body, I began working a full time job that has me sitting in front of a computer all day long. I also have two children still in elementary school that are active in extra-curricular activities, am working on completing my doctoral degree in my "spare" time, am trying to improve my French (it will greatly help my career path), workout 6 days a week in my basement (thank you Cathe!), and instruct 2 evening fitness classes a week. I would also like to refresh my piano skills and am considering taking lessons in the fall. I find it the height of gall and judgement, not to mention rude and disrepsectful, for any one of my class participants to dismiss my classes as ineffective or "not good" simply because I am at 24% BF rather than 18%. They do not know the stresses and time challenges I face in my "real" life, and have no right to sit in judgement of my physique. I am very good at what I do; I have excellent motivational skills, and a vast amount of knowledge and experience that leads to routines that are intelligently designed, balanced, and leave the participants feeling successful. THAT is how an instructor should be judged.
So, to summarize my rant: NO, please do not apologize for how you look, or limit yourself because of it. If others are going to judge your class based on your personal BF level, then they are the ones who are going to miss a great opportunity, and it is their loss. Do not wait until everyone else has given you permission to do something you want to do. There will always be naysayers; focus on your goal, ignore all the Negative Nellies, and go for it!