Brolish - you're right in that the differential between MP and easy training pace tends to get squeezed closer together as marathon time increases. But there has to be a limit to this. It cant be right to be doing LSRs at, or perish the thought, quicker than MP surely.
So MP+60 should be close enough for anyone. I have always been puzzled by runners saying that they cant run at 10 min/mile because it feels too slow. I can happily run at 10 min/mile as can Jocelyn Payne, she mentions it in the interview I linked to a while back, so if sub 3 runners can run at 10 mins/mile why cant 3:30 or 4:00 runners?
Another element is that the faster the marathon time, the higher the number of long runs completed and the more they are run on tired legs. The flip side of that is that the slower the marathon time, the lower the number of long runs and the fresher the legs when longs runs are tackled possibly leading to faster relative paces.
But, in general, slowing it down and getting more long runs done at a slower pace, prreferably on reasonably tired legs, should lead to improved marathon times and a better race day experience.
Edited: 05/02/2013 at 12:18