Briefly, I made the decision to drop out around the 18-19 mile mark and walked to the nearest aid station due to extreme nausea and cramping. I was very concerned that even if I could get past the nausea, I might be risking long term damage to my muscles, as I forced them to cooperate against their will. I had very marked goals going into the race, and I believe that it was more important for me to live to run another day to achieve those goals than it was to complete and make it to the finish line.
Yes, I could have finished this marathon. But, at a minimum, it would have cost me an entire marathon training cycle. Stopping when I did will allow me to spend this training cycle at another time when my body and the conditions are more cooperative. I made my decision that continuing today was not worth the cost to my body, and I can more quickly leverage my fitness to cash in on my goals in the nearer future.