Pulling up a map with a scale, it's about 2600 feet (as the crow flies) from the middle of T4 to the middle of T8. Factor in the fact that the walk (particularly once you get to T6) isn't dead straight, it's longer than that but I doubt a mile. That number must include significant walking within the terminals at both ends. The middle of T4 to the middle of T6 (which is about what the tunnel does) is about 1400 feet.