I smoked 1.5 packs a day for years. I stopped quickly, took up running to keep me from going back, and got very seriously involved. My peak year included 2,600 miles. I did a lot of marathons and between 20 to 30 races annually. I ddin't get involved in cycling until later.
Stopping is the hardest thing I have even done. People say the last six miles of a marathon is tough. It is but the discomfort doesn't last long. Stopping smoking is uncomfortable for a long, long time.