The whole downhill thing

I started to have some pain in my left quadricep. I had been anticipating pain, but it was knee pain that I had been worried about given my injury from my first marathon. I was perplexed. Why was my quad hurting? Where was this odd pain coming from around 11 miles into the race? Had I injured myself inadvertently and not known it?
I kept tracking that woman, keeping pace with her: she was my beacon. I was still on track for a sub 3:30 marathon, which was somewhat typical for me. I tried to remember how long I had kept that pace for during the Victoria marathon, and I seemed to recall that around mile 17 I let it go and started to slow down to come in at 3:39.
I did a double take crossing the halfway mark. A 1:42 half marathon? I beat my PB half marathon time by two minutes and certainly my marathon pace is slower than my half pace. I started trying to do math in my head, as one does if one is attempting to make life difficult for one's self.
I had two paces to adhere to: the pace I needed to run a mythical 3:30 (I always keep this pace in my head for fun); and the pace to run a 3:40, which is typically what I manage. I started doing such fun calculations as: "where should I be time wise if I'm running an eight minute mile at mile 12?". I swear. the further you run the more you cannot think straight. By the time I discerned that 12 miles at an eight minute pace should take me ninety-six minutes (which is an hour and thirty six minutes) I was closing in on mile thirteen. It was horrible.