dog identification

So me and m fiancé got our little doggy in August. She was in a really bad home that couldn't take care of her anymore. They didn't care for her at all so they never really inquired as to what kind of breed she is and they didn't tell us how old she was either. Regardless we love her. But...

Rescued her from a shelter a few days ago. SO SWEET and gentle all the time. very calm inside the house and walks great on a leash but if you tell her it is ok to run - man does she run! What breeds are in her? Everyone says pitbull right away but what else? She is pretty tall and pretty thin...