If you actually watch Truman in the movie, you'll notice he had already started building his escape tunnel. The entire movie isn't him learning everything is fake, it's him trying to stall everyone while he figures out how to get out. He's testing the boundaries.
He escapes through the tunnel they find. That's how he gets to the dock to steal the boat. It doesn't show him in the tunnel. He's just gone for awhile evading everyone, and eventually they find the tunnel in the basement.
The world's masturbation record cannot be set because it's a race to the lowest number of strokes and 0 was recorded in the year 832 AD by one Hywel Beton.