I dropped out of a marathon several years ago with a slight hamstring pull, hoping to run another one several weeks later to qualify at that one. It didn't work...the hamstring got worse before it got better. I don't know if you can second guess yourself because sometimes the injuries are more severe than we think, but due to the adrenaline, the endorphines, and the "marathon attitude", we don't feel them so much. Could be the damage was already done bu mile 13. Good luck on healing, I hope you're a better, more disciplined "patient" than I am!