Try to solve this - Mr. Pi and Willy

Every morning at 8 am Mr. Pi walks to his office that is 6 miles away. His dog Willy accompanies him. Willy knows the way and runs ahead with twice of Mr. Pi’s speed. When he reaches the office, he turns around and runs back to Mr. Pi. Once Willy reaches Mr. Pi, he turns around and runs ahead to the office again, where he turns around and runs back to Mr. Pi again, and so on. They both reach the office together at 10 am. How far did Willy run?

(modified from Physik, Gerthsen 7th Edition)