I live in the western part of the US (Colorado) and 600 miles doesn't actually take that long to drive. I regularly drive to my parent's house - which is about 550 miles - in 8 hours. I'm essentially driving the speed limit the entire way with no slowdowns except for one or two gas/restroom breaks. I would imagine driving that distance in GB would involve a much lower average speed.

I have always assumed this is why distances in the US (reasonably far away from the coasts) are "not that far".