A more upright position will easily accomodate any extra weight you carry around the middle. Just don't worry about speed for a long time yet, enjoy riding and the scenery! People will pass you all the time. As you lose weight and gain fitness you can start thinking about a bike with a more forward-leaning position, if you want to go faster. But as I read somewhere - there's always someone out there who is fitter and faster than you, so there's no point in worrying about it. Riding slowly has its own special value.