1. Overtraining & Undertraining
It’s true that to achieve a personal record at any distance requires executing new strategies—from increased mileage to marathon-specific workouts to even a different, but well rehearsed, fueling plan—but showing up at a marathon start line without an adequate-for-your-body taper, sore and taxed legs, an injury, or ill-stocked glycogen stores will almost guarantee a bonk.
And while toeing the start line a little undertrained may end up working out for some more experienced distance runners, too few completed long runs will make the last 10K of the race much more painful than it needs to be. In short: Respect the distance.