Is infrequent cardiovascular exercise (like jogging once in a while) worse than not exercising at all?
By and large, exercising occasionally is better than never exercising at all. According to a telling 2004 study, men who worked out only on weekends - the stereotypical weekend warrior - were less likely to die prematurely than those who remained sedentary.
They also can attain a surprising level of cardiovascular fitness. In a 2006 study of out-of-shape adults, those who began vigorous endurance training on weekends were as fit after 12 weeks as those who worked out more moderately five times a week.
But of course there are downsides to sporadic exercise. Weekend warriors tend to suffer acute tears and sprains more often than consistent exercisers (though are less likely to develop overuse injuries). A more serious concern involves the heart. Like any muscle, it can be overtaxed by sudden, unusual demands, so people who exercise intermittently may put themselves at a relatively high risk of a heart attack compared to frequent exercisers or even to themselves when sedentary.
A major 2011 examination of activities that can cause heart attacks found that regular workouts reduced a person's overall risk of cardiac arrest, but any single bout of exercise, especially by weekend warriors, increased the risk of a heart attack at that moment. As the review soberly concluded, "Acute cardiac events were significantly associated with episodic physical and sexual activity," but "this association was attenuated among persons with high levels of habitual physical activity."
So, for your heart's sake, make physical and sexual activity a habit.