I think the Robin Hood (Nottingham) marathon is tougher than it looks. It's certainly flatter than it used to be but it's a long, lonely run in from the split with the multitudes doing the half at 11 miles. If you're running around 3 hour pace than you could find yourself pretty much alone for most of the second half and there is much less support from the crowds. Also the section around Holme Pierrepont at 20-22 miles is as monotonous as hell and often very windy.