LOL at everyone acting like eating an unfrosted pop-tart will be the death of you. It's a pop-tart....I have no issue with my children eating a pop-tart as long as they stay active and both of them are very active in sports including soccer, softball, and dance.
Eating one pop tart every once in a while is basically harmless but eating one regularly is when it becomes a problem. As others have mentioned, exercise doesn't replace a healthy diet. If you ate McDonald's at every meal and exercised 4 hours a day, your health wouldn't be great.