A 747 gliding all engines out and not immediately above an airport is kind of a major crisis. The number of times that has happened is an extremely rare occurrence.
On average and handled correctly, a 747 at 40000 feet has about 100 miles of range, discounting winds.
Which is not a lot, if the only airports within 100 miles have runways too short for a 747.
What about a body of water? It’s not ideal but it seems like the best option if there’s no suitable runway.