i smoke out of habit now, i started when i was 14 or so. I think it had to do with my dad being a big smoker too. Now i smoke between half a pack and a pack a day. Usually a cig will calm my nerves, but Im guessing my nerves were rattled because I hadnt had a cig for a awhile... sooo. It is hard to quit too, ive tried many of times. Now I just do care. I excercise everyday and I am a pretty good runner and I can pass all the Armys physical fitness test easily enough.
But ya smoking sucks I wish I wouldve never started, and it will probally take a scare for me to quit permanently.
The key difference between you and MOST people is they smoke and sit on their ass so it stacks up against them. True you probably can not run as much as you should be able to without smoking but at least you are doing physical activity. You are still better off than most chain smokers I would think.