From a purely psychological perspective I think there's always a case for going 'over distance' in training and have done so numerous times in half marathon training.
I've never gone beyond past 23 miles in marathon build up. I've ran 7 marathons and have had a similar experience as you describe in 2 of them. I'm not sure if me training further than 26 miles would have changed this though. There are so many variables in a marathon race that can affect performance and can spark a chain reaction of negative thoughts which may lead to a melt down.
I think to have any psychological benefit you'd have to run 27 miles regularly, so that the actual 26.2 race is the norm. Bit of a simplistic answer, but that's how I see it.
My first little run post-London today. Did a recce for a forthcoming trail race along IoM coastal footpath. Spent a lot of time in the car, looking at map, investigste a small section on foot, back in car and so on. Nice weather for it though.
Best of luck, AWC, Ramjet and Hilly. Go like the clappers!