Is it possible to exercise too much?

An hour a day of exercise is great! Keep it up! Make sure you get plenty of rest though. You can overdue it. Make sure you stay hydrated. Plenty of water. No really, lots of WATER. Cross country running or swimming would be a great activity for you. Be mindful of what you're eating. All those things are ok to have but try not to turn junk food into a staple diet. Be careful about the workout tapes. Some aren't meant for a young lady of your age and you may hurt yourself. Stay safe and happy!