hi Ray, welcome to karateforums. For all I'm not a TKD student there have been many people like yourself join our class (kickboxing) which also relies a lot on kicks, and with a little bit of work (which everyone has to do regardless of their shape and size) anybody can practise a martial art, IMO. Being able to kick well has nothing to do with your physical state, but more how supple your muscles are, which comes with practise. The only reason the instructor you mentioned was so "thin", as are most MA practitioners, is because the class gives you a workout and it's addictive which means you practise it, burning fat. If you want to do it I think you should go for it, the instructor will only go at your pace and won't force you to kick chin-height if he knows you can't. Hope I was a little help there.