I have heard many references to "the wall" that many runners hit @ mile 20 of 26.2. I always assumed that to mean that runners become aerobically fatigued around this point of the marathon and have to slow down.
My experience has always been one of purely muscular fatigue (mostly bilateral quads and hamstrings) that make every step painful. I am never aerobically fatigued or out of breath and I never have trouble mentally. Every step hurts and it feels easier to run uphill than it does to run downhill. This despite adequate hydration, water at every water stop, and gels 15 min before, at mile 5-8 (slowly taking it in b/c they make me sick sometimes) and mile 15-18.
I have rarely had luck pacing in a marathon because this always happens (even if I keep the first 10 miles within 10 sec/mile of goal pace). Inevitably I feel wonderful at 13 miles and just afterwards my calf and quads start to hurt and then its a predictable decline from there. Is this what the wall feels like for everyone. Is it always purely muscular?