Yikes...
Simple answer is a mile is a mile, high or low, and journey time is simply journey distance/average Speed over the ground (aka ground speed).
If the aircraft can maintain the same ground speed at both 500 feet and 50000 feet then journey times should be the same. I never flew the Norfolk land shark but I would have thought it could maintain a high subsonic ground speed of say 500-550 knots at both 500 feet and 50000 feet.