How tall is your husband? I'd want to borrow his Roubaix instead (I LOVE my Roubaix ). I'm 5'7" and I ride the 54cm.

The regular stationary bikes at the gym are a bit of a different workout. Spin class bikes would be a little more like it. I'd say just get out on the road and pedal. Try the park. Start out getting used to things and warming up on the flat parts. The hills may be quite challenging, but get accustomed to the gears and give them a try. You don't have to fly up them. Make it your goal just to keep pedaling all the way up. Keep in mind that they will just get easier the more you ride, because you'll get stronger and you'll keep shedding the excess weight. Use the gym for weight training and when the weather is bad. If you're just doing 30min workouts now, then start out with some short rides, particularly if you do those hills. Keep in mind that you may not feel quite as bad outside as when you're sweating buckets stationary in a gym. You can also stop and take a breather whenever you want. Just go out and ride until you want to quit. Then look at how many miles you've gone (rather than at your time at first), and use that to guide you as you work to improve your fitness.