I feel like a fraud
Why? You’re not making any claims about yourself. You’re simply helping to run a class. I’ve known high-grade black belts that were not fit to be around other human beings. A black belt doesn’t make a decent teacher. Commitment to knowledge and the growth of others does.
Anybody can become a sensei – it’s valueless
I realise that some regional instructors give out sensei belts like water, and that makes the position seem less special. But the truth is; each person brings their own value to the role. It doesn’t matter who else is a sensei; it’s how you behave as a sensei that matters. Some people are fantastic senseis, and others are not. Interestingly, it’s often people who become senseis for the wrong reasons who quit early. Short term gratification or respect is not enough compensation for the hard work that it takes to become a good martial artist, let alone, a good sensei!
I don’t drive and can’t get to sensei class
It’s true that sensei classes are often located further away than your regular class, but how does your sensei get there? If you get there by public transport until you graduate, there is almost guaranteed to be someone else going that you can get a ride from after that.
I don’t want to be bullied into taking a class
Then don’t take a class. Make it clear from the outset that you are happy to help out, but you don’t wish to take your own class. If you’ve been doing karate long enough to be invited onto the sensei program, then you should have enough assertiveness not to be pushed into anything that you don’t want to do. Think about that for a moment... What’s the worst that can happen if you decline your own class? The sensei will say “Please please please” then look at you with Puss in boots eyes. Steel yourself and say “no”, you wuss. ;-)