I have run several marathons including a 2:30 so it's not like I'm new to this. Today I flew through 18 at 6 minute pace before completely tanking at 20. I dnf'd for the first time in my life. I have never had the bonk set in so quickly or violently. Even when I do crash, I have always been able to limp in to the finish at 8 minute pace after walking some. All of my pre race training indicated I could handle this pace as I have in the past. Race conditions not perfect but not enough to merit a blowout? Why did this happen?