A friend of mine (who has run ~5-6 marathons) told me that he knew he was ready to achieve his goal time in the marathon when he ran a 23 miler at his projected goal time.
So his goal was sub 3:00:00 (6:52 pace)...when he ran a 23 miler in 2:58:00 while in training (7:44 pace) he knew he was ready. Sure enough, he ran just under 3 (2:59:20).
My goal is under 2:30. I feel that a 23 miler at 2:29:00 (6:28 pace) would probably be a taxing workout, but not difficult. Has anyone ever heard of something like this?