Hey Saguaro,
If quitting smoking is so easy, and you have done it a thousand times...why pick the habit back up?
Stopping smoking is a lot more difficult than most people give it credit for. Nicotine is highly addictive, and your body readily forms a chemical dependency to it. So when you do quit, there are alternatives to doing so cold turkey, which can really suck. Nicotine replacement therapy, in conjunction with a little bit of cognitive therapy (aka, what activities make you smoke? do you smoke when you are stressed? what can you do instead of smoking?) greatly increase your chances of quitting for good.
Before I talk about diet and exercise, I think it is very important that you first see your general practitioner before beginning any new diet or exercise regime. Your age and weight puts you at risk for conditions like diabetes, high blood pressure, and high cholesterol- these all require careful management and observation. Overall, diet and exercise changes WILL benefit you, but he or she will be able to tell you how to do so most safely.
As far as your weight loss, I agree with Chris. Dietary changes are the most effective way to lose weight. By keeping to an 1800 calorie diet, low in carbs and high in fruits, vegetables, you can adopt healthier eating habits easier than you might think. One of the best things I can recommend is to carry a reusable water bottle with you, and make sure you drink plenty of water...and just water. Soda and alcohol do you no good. Plus, proper hydration reduces cravings and helps you feel more full at meal times.
With regards to physical activity and exercise, you are definitely not alone when it comes to being apprehensive about starting an exercise regime. Take it slow, but be consistent and exercise within your boundaries. By doing too much too soon, you are at risk of hurting yourself, and discouraging yourself from exercising more in the future.
Here is a great page from the CDC with all sorts of tips and guidelines on how to start exercising, and how to do it healthily.
http://www.cdc.gov/physicalactivity/everyone/guidelines/adults.html
Good luck, and keep us posted on your progress! It's never too late to start making healthy decisions
