Listen to your body. If you haven't been active for a long while, it can be stressful to suddenly spring into constant action. Start with a few short rides a week; increase either the frequency or the length every week till your body tells you to take it easy.

As for food, dieting and increasing your exercise level at the same time can be very difficult. Do *not* try a carb-restrictive diet when you're increasing your activities; you need the carbs for energy. I found, though, that cutting back on fat helped me tremendously. There are a lot of yummy low-fat recipes out there. It's almost impossible to eat out and eat low-fat and still get the nutrition you need; this is a constant struggle for me. Much better to prepare foods yourself, if you can find the time.

I'm not a doctor or even an expert; these are just my thoughts.