There are two ways of looking at most things and I would like to give the other view an airing: I am not saying either is right or wrong but there are a lot more people who read this thread than there are who post in it and I feel it right to debate the alternatives.
When you read about marathons, a lot that is written says the race only begins at 20 miles. I know what this means but I prefer to think of it in terms that by 20 you know where you are. The majority of people who get to 20 feeling fresh as a daisy will get to the end with no trouble: if you get to 20 and are dead then there will be very few who get better over those last 6.
Most people will have run 20 miles or so in training, once or maybe more. So what is it about these last 6 that drives people? I have gotten to 20 miles before and stopped when I knew that my target was not achieveable, I have no desire to injure myself when such an injury could mean I couldn't run for a year or so, maybe more, maybe never again. When I have stopped I have been able to adjust my race schedule and set (and achieve) new targets. On Sunday I was aiming at sub 3:15 to get GFA for VLM. If I had got to 20 miles much over 2:30 I would have pulled up at that stage. There are people who have run marathons before and will run them again who did pull up on Sunday and they should be respected for making a wise long term decision.
So in a nutshell, if your aim was to complete at any cost and you did then well done! If you had a bad day at the office and knew things were not to be and jogged in or pulled out then also well done! People should be congratulated for being wise as well as gutsy.