Matthames said:
Welcome to the world of internet forums

It is perfectly normal and acceptable practice on internet forums not to reply straight away.
Apologies Matt I zipped though the posts rather too quickly it seems missing your response at #7.
I would contact your future employers for an informal chat about the day and suitable dress. I would wear
clean cycling gear, long black leggings and a hi-ish viz top/jacket to show you are half serious about safety, plus helmet of course. I guess if you are arriving by bike at this time of year you will have your other warm gear such as neck fleece, hat and gloves and maybe a pannier carrying stuff which all shows you are a proper cyclist. I would clean and service your bike. Attention to detail helps. I think if it were filthy that would raise eyebrows and perhaps create the wrong impression.
Apart from be yourself lots of luck, you probably won't need it. You've obviously applied for the position as you believe you can do the job and they've called you to interview as they think you may have the qualities and skills to be a cycling instructor. All you have to do is show them that it is indeed the case.