How Early was the Plane?

A plane is delivering the mail to a remote outpost. A mailman leaves the post office on his motorcycle to meet the plane at the airport when it is scheduled to land. The plane arrives early and to be as efficient as possible, the mail is immediately sent toward the post office by horse. The horse meets the mailman on his motorcycle a half-hour after leaving the airport. As a result, the mailman returns to the post office 20 minutes ahead of time. How early was the plane?

Pony Express
minutes