I would not worry too much about your heart health at the moment because if you are anything like my daughter who is always hyperactive, you are exercising your heart at a good level already. I also find that the P.E lessons in school and extra curricular activities are great opportunities to get involve and keep fit. If you are a relatively active teenager then i would not worry too much. What I would advice is to eat healthily because what you eat now will affect you later in life 20-30 years down the line. This is how long it takes to block your blood vessels. And this happens silently so you would not be aware of it but all the burgers,chips, pizzas and junk that you eat will come back to haunt you in adult life, or simply give you a heart attack.