Interesting...

Despite being a contributor in the Training thread, I know little of the science behind it. But a very good marathon runner from our club (2:15 or some such unbelievable time) advised against running 'long' races in training for a marathon, i.e 20 mile races. He said concentration should be more on the quality sessions and you need to be running consistently hard and fast, and the 20 mile effort would take too much out of you.

But he, and you, are far faster than i'll ever be, so take it with a pinch of salt. I know he ran a 2:15 marathon one week, followed by a 2:17 the next, and had a 10km time of 30mins-something, so i'm sure he knew what he was talking about.