I just want to add - this is such an inspiring thread! and I hope you both have better luck finding good bike shops that can set you up with affordable bikes that work for you. They certainly exist, but not every 16 yr old backwards-capped bike store help would know.

I think maybe the biggest obstacle to losing a lot of weight is the same as getting much fitter or getting much faster on the bike - putting in the time. The effort is one thing, but you really have to be willing to put in lots and lots of hours over a long period of time, because biking isn't that intensive a workout. The upside is that that's a whole lot more fun than putting in hours at the gym Commuting by bike is a great way of sneaking in miles. But that's why you really need a bike you're comfortable on, and some nice places to ride. You'll be plain miserable if it feels like a chore.