The importance of exercise is clear. But you're young, and I highly doubt you want to do the regular gym stuff that adults do. There's another way that I think you'd enjoy. Games! I know you love being outside and you have a very lively imagination, so I thought this would be a good idea for you.
You can just run around and play whatever comes to mind. It can be a world where aliens are invading, or you're a cowboy, or maybe you're an adventurer. Or you can play games like soccer, baseball, basketball, or something else. Whatever gets you moving.