How big a difference?
A huge difference. After smoking for 25 years I was unable to even walk up a couple flights of stairs without getting winded. I finally quit about 2 years ago. Like many others, I put on a few pounds over the following year or so. About seven months ago I made some serious changes in eating habits. I also spent a great deal of time studying the body's energy systems, nutrition, training and adaptation. I started working out seriously and bought a road bike a month or so later. I am nearing the end of my second round of P90X and ride about a hundred miles a week. I am in better shape now than I was in high school and college. Your body will start healing as soon as you put you them down. I am still astounded every day how different my life is now. I am training hard and hope to be strong enough to try some racing later this year. If I can help you in any way, let me know. Good luck. Yes it can be done and yes it's worth the trouble.