My last 5 Marathons I've experienced the same problem. What limits me in these races has been my legs getting sore as the race goes on. This happens early on in the race usually even before the halfway point. I never hit the wall and usually come close to even splits. I feel like if I find a way to beat this problem then I can run way faster.
My training consists of 80-100 miles per week. I do progression runs, tempo runs, and threshold running for workouts. The funny thing is I really don't experience this kind of soreness during my workout sessions.
I even experienced this during my best Marathon of 2:39. I was sore before the halfway point and thought it would be impossible on that day to break 2:40 but I never slowed. I just could not pick it up any faster. I'm not limited by my breathing or my energy levels. I just have no fight left in my legs during the later miles. Downhills late in the race are not an advantage for me. I'm so beat up at that point that the uphills are almost as fast.
Has anyone else experienced this? Would more miles per week do the trick? More MP runs?