You need to file a petition for a restricted license in the Circuit Court of the jurisdiction you were convicted. The forms are available at the Clerk's office or online at the Virginia Supreme Court's Website. There are several steps you need to take so it most likely will be faster and with a better chance of success if you hire local counsel.
First of all you need to appear at court. If you fail to appear the court will not decide the DUI case but will issue an arrest warrant. The maximum fine is $2500 plus court costs and up to one year in jail. You need to hire local counsel and prepare your defense.
The officer hass to prove that you were driving on the Commonwealth highways. They can do this by direct or circumstantial evidence to prove their case. It certainly sounds like the government may have a hard time with this case from the facts presented. However there are two sides to every story and their are factsthat we do not know about yet. I would suggest you contact local experienced counsel immediately.
Contact a local attorney in Sterling to help you with the expungement process. If you choose to try to do this yourself, the documents can be retrieved at clerks office with an explanation about how to proceed or you can go online to http://www.courts.state.va.us/caseinfo/home.html and print the form.