Excercise can prevent heart disease but once youve had a heart attack its too late for excercise to you

No this is not true if you start to exercise after a heart attack it can help prevent any other heart attacks

It's not too late to exercise after a heart attack. After the heart attack, the patient can receive therapy such as device (pacemaker, implantable cardioverter-defibrillator or cardiac resynchronization therapy) and drugs (beta blocker, statin, ACE inhibitor). They are also advised to have some moderate exercise regularly to increase their exercise capacity. There is also heart disease due to exercising too much as seen in hypertrophy cardiomyopathy

I'd say replacing that habit (of smoking) with a more healthy habit, like going to the gym. c;