You can definitely make a big dent in your net calories by using exercise, and you don't have to be a professional athlete to do it. Assuming a 2000 calorie diet, that's 14,000 calories a week. I've been working out on a stationary bike this winter, and the training tools say that I burn about 600 calories an hour. Let's assume that estimate is a bit high, and I actually only burn 500 calories per hour. If I do 2, 1 hour sessions per week, I burn an extra 1000 calories. That's 7.4% of my non-workout calorie expenditure.
Sure, you'll have a hard time getting to 2 times the regular energy expenditure with exercise, but eating 2 times your baseline energy requirements is a terrible diet anyway. Exercising can provide you with a little wiggle room for when you want to eat a few extra calories as a treat. Eating only the bare number of calories makes for some pretty uninteresting eating habits. Also, cardiovascular exercise is important for other health reasons. If you just eat the right number of calories and never exercise, you will still be in bad cardiovascular health, and will still suffer health problems because of it.