There is a lot of excellent advice in this thread!

Something else that you might want to keep in mind is that biking is a very unique sport. There are no limits to ones ability to bike just based on size. Because of this, there are lots and lots of stories about people (men and women both) who started out heavy and through their developing love of biking, are now much thinner. Some of them are now racing, starting to run, doing triathlons, etc.

With the exception of the typical jerk (and there will always be those types out there), I would imagine that most cyclists (particularly those who work in bike shops and presumably know tons of other cyclists) know of at least one person personally who started out biking when they were very large. In fact, one of my riding buddies from my old job was that way...he started biking at well over 300 lbs. By the time I met him (and started riding myself) he was down over 65 lbs and still losing.

I guess my point is that while you'd imagine that most of their customers are skinny little racer types, that's not likely the case. Even if it is, that doesn't mean that they don't know of a bike fitness success story first-hand. You may not be as out of place as you feel!

And good luck - to all of you soon-to-be cyclists...you're gonna love it!