This resounds so much with me right now. I just finished Rock n Roll DC two weekends ago, and hit the wall almost exactly at mile 20. Part of that was course design, in that the majority of elevation gain was in the last 5 miles, but I believe a lot of it was mental. Around mile 24, a pacer for 3:30 showed up on my side, and I was devastated because I thought I had a good buffer. But just seeing him was enough for me to kick it up, and went from a 9:00 min/mile back down to a 7:30 min/mile. Finished in 3:29:38 and had trouble walking for a couple days after, but worth it.