I am a smoker since last 35 years, most of those at 40 a day. I gave up in the mid 90's for 5 months and those five months were the most blurry days of my life, couldn't think straight, did not know most of the time whether I had just switched on the computer or just finished my work and switching it off, needless to say I felt like a vegetable.
I had a lot of support from everyone in the office including my boss, but I felt horrible and started back on and have continued since then.
I have cut down since last year or so to 20 a day and trying hard to manage to get it down to 10 a day. I am 51 but finding it hard to keep pace with 71 year old non-smokers, yes it does kill you slowly there is no doubt about that.