Oh man, we did flyball with Kimber when she was about 4. She was good at it- ball? Check. Turning? Check. Drive? Check check check. Like Penny, she got really revved up watching the other dogs run. The barking! The lunging! The "I wanna chase too!" I actually had to get DH to come to class with me because I wasn't sure I could control her.
We decided to stop after that six-week class because she would hit the box hard on the turn and she'd limp the next day. That was nothing we wanted to fool with. For the next series we returned to agility and found our home.
Hope you and Penny have fun!