(I don't want to take away from your fine effort with more of my rants about people who use Garmins in marathons, but you do know that your difficulty is easily cured by simply wearing a lap counter watch.)

Yeah, yeah, I know but I have three problems with this:  1) sometimes the markers are small and easy to miss, especially if the course is crowded, 2) sometimes the markers are right at water stops and are easy to miss because I'm distracted taking care of hydration, 3) sometimes I space out while running. 

I tried hitting the lap button on two separate (shorter-than-marathon) races and screwed up both times.  Lesson learned.