I smoked about 1/2 a pack a day for three years before I started riding, about a month into seriously riding, I decided to quit, and it made a world of difference. You'll feel about a million times better every day and you will definitely notice difference in your riding. It was even in the first week that I noticed a difference in my stamina and increased lung health, on and off the bike. Mind you, I was only 18 when I quit, the older you are, the longer it'll take you to rebound from the damage you did to your lungs, stop now before it becomes harder to back track.

On another note, the easiest way to quit is to convince yourself that you really really want to quit, and if you want to improve your riding, feel better, and live longer then you want to quit smoking.