If you've smoked for a long time then you'll have forgotten what it's like to feel on top of the world. Smoking pervades every cell in your body, it gets into the structure of our genes and can change them for future generations. Carbon monoxide bonds to the same receptors as oxygen so starves usof the life-giving element, and consequently the ability to process the energy in glucose from food efficiently leading to fatigue and lack of energy. And at the same time as introducing it's own toxic effects. It thickens blood until capilliaries become blocked & broken, starving our skin of any nutrients our blood has still managed to carry about & aging us before our time. More to come.
Within 20 mins of your last cigarette your blood pressure & heart rate are back to normal. After around 8 hours your lung function will increase as carbon monoxide levels in your blood fall. Within 24 hours your risk of a heart attack is falling. After 48 hours, nerve endings damaged by the habit start to re-grow. IN just 10 days your circulation is similar to that of someone who's never smoked. In 1-3 months the risk of heart attack has reduced substantially. After just one year risk of you suffering from heart disease is around 1/2 of it's previous level. And from 5 years on your risk of a stroke is roughly the same as someone who's never smoked.
Whatever age you are now then you're going to feel a hell of a lot better than you do now when you stop. In the west it's almost accepted, sadly, that we lose our fitness as we get past middle age, or for a lot, anywhere near it. It doesn't have to be that way. I personally know a triathlete in his 70's (he doesn't smoke, obviously), and he says he's not unique - there are several others older than him still competing. Now you don't have to go to that extreme to feel great and for your body to do what it was designed to do, with reasonably good nutrition (it doesn't have to be strict) and very light exercise using your whole range of body movement (which will increase the more you do) then you' can feel great physically and also mentally sharp well into old age.