I agree that's hard advice to act on. Nor do I feel it's necessarily good advice... at least not for me. I find I go through cycles of high and low energy during the course of a marathon. If I coast through those low energy times, I can usually make up for it when my energy returns again. This tends to coordinate with my food intake during the race. The best marathon I ever ran was one where I went out way too fast because I felt so good. I rode that wave until it passed and then settled in for something more reasonable, but it bought me enough to finish me 15 minutes ahead of schedule. For me, I find it best to do what feels right, or else I get hung up and don't perform as well. Perhaps that's just the psychological piece of it all.
But I'd be interested to hear any studies that show keeping an even effort is better for performance overall than riding the up and down energy wave.