Run/walk was, for me, a game changer. Physically, it was easier to drink/eat without choking. The 'rest' was always welcome (although you're actually speed walking...). Psychologically, being able to work in 10 min blocks was a revelation. If the previous block was bad, then this one will be better, knowing that you have three songs before you can switch off for a minute etc.
I ran my long training runs continuously, and by the time I was at 18 mile distance, I didn't think I could mentally do it (I never doubted myself physically). Someone suggested the run/walk thing and I know it's bad to switch plan so late on (a week before) but it felt right.
I worked out my ratio of run/walk, based on my desired finish time (sub 4:30), my conversational/easy effort run pace (6 min km) and my walk pace (8:30 min km). It took a few scribbling calculations to get to 9/1, but it was a good one because it was easy to know when my walking minute was due - whenever my watch said 9/19/29/39 etc mins.
I finished in 4:27 and still had a sprint finish in me. And really no after effects. I feel worse after an hour of weights than I did after that marathon. Average pace was exactly 6:15 min kms, which is a slightly faster average than I was hitting on long training runs (6:20-6:30 min kms), even with the added walking sections.