On this below, I couldn't resist. It surprised me recently to learn how quickly the body can recover from decades of smoking:

20 minutes after quitting: Your heart rate and blood pressure drops.
(Effect of Smoking on Arterial Stiffness and Pulse Pressure Amplification, Mahmud, A, Feely, J. 2003. Hypertension:41:183.)

12 hours after quitting: The carbon monoxide level in your blood drops to normal.
(US Surgeon General's Report, 1988, p. 202)

2 weeks to 3 months after quitting: Your circulation improves and your lung function increases.
(US Surgeon General's Report, 1990, pp.193, 194,196, 285, 323)

1 to 9 months after quitting: Coughing and shortness of breath decrease; cilia (tiny hair-like structures that move mucus out of the lungs) regain normal function in the lungs, increasing the ability to handle mucus, clean the lungs, and reduce the risk of infection.
(US Surgeon General's Report, 1990, pp. 285-287, 304)

1 year after quitting: The excess risk of coronary heart disease is half that of a smoker's.
(US Surgeon General's Report, 1990, p. vi)

5 years after quitting: Your stroke risk is reduced to that of a nonsmoker 5 to 15 years after quitting.
(US Surgeon General's Report, 1990, p. vi)

10 years after quitting: The lung cancer death rate is about half that of a continuing smoker's. The risk of cancer of the mouth, throat, esophagus, bladder, cervix, and pancreas decrease.
(US Surgeon General's Report, 1990, pp. vi, 131, 148, 152, 155, 164,166)

15 years after quitting: The risk of coronary heart disease is that of a non-smoker's.
(US Surgeon General's Report, 1990, p. vi)