Dude...I smoked from 14-25, some of it less heavy but at least a good 3 years where it was about a pack a day. Had a few health scares so I decided to quit smoking by running...twice... The first time I had never run before but didn't have the aerobic capacity, so I started by walking 5 minutes, then 10, and so on. Eventually I was finally able to jog(like 14min pace) for 30 minutes and then was able to do other exercise. Work and other stuff got in the way for a few years and I fell off of running for a bit and unfortunately smoked intermittently during this time too, maybe 5 cigarettes a month or something. Then I picked up running again 3 years later seriously by doing c25k and that worked great but I felt like I was running really slow. I still ran but wasn't really training for anything. It's about 2 years after that and I am now training for my first marathon in December. My mile average pace has went from 14:00 to 9:30 over the course of about 5 years, I only really ran for about one and a half of those 5 years though. I think I am a very slow runner but I hope to BQ someday...

Take care of yourself dude/dudette, it'll take time but I think I started smoking fairly early(I'm pretty dumb) and I can keep up with some of my friends who never smoked in their life. Also, almost every doctor I saw along the way told me that basically it's ideal if you quit before you are 26 but as long as you quit before you are 30 there is a very good chance you can heal the majority of the damage provided you have a good diet etc.

Also YMMV but prepare for phlegm...a lot of it...weird kinds of it...