I quit smoking when I was 34 ( I'm 56 now). At the time I was living in a hilly-to-mountainous area and commuted about ten miles to work by bike a couple of times a week. Several things happened within the space of one week that led me to quit: I split up with my wife, I finished grad school, and my father was diagnosed with cancer (he lived another twenty years). Everything was so completely topsy turvy that the stress and emotional roller coaster of kicking the nicotine habit kind of got lost in all the rest. I did drink a lot for a while: alcohol and coffee intake both skyrocketed. The fact of the matter is, though, that after having tried to quit several times, that time it stuck.
I do remember that I was expecting to start feeling better immediately and it didn't happen. Especially while riding, and especially while climbing, I coughed up years' worth of crud. I suppose that without any new smoke and tar pushing it back down into my lungs, any exercise and heavy breathing helped my lungs expel what I'd been pumping into them for years. It took a month or two before I actually started feeling better, but it did happen and in the last 22 years I have hardly ever been tempted to start again.