Congrats on doing a half hour the first time out, Meg33! Welcome to the forums!
Stamina & improved fitness is a personal thing I think, but you'll feel better fast if you keep at it!
I'm not an expert on weight loss through biking. In fact, I never had a weight problem until I started cycling six months ago. I know what the problem is...I eat far more than I burn off now, but can't seem to stop! If you can keep your food intake the same as you have now, you'll see results pretty quickly!
Good luck to you!
Sue