Finishing a marathon race needs proper preparations
As a runner, there is little you can do to significantly improve on your fitness level three weeks out from a marathon race, but there is a lot that can happen to ruin your race day—or even your reputation, if you are an elite runner and happen to stop at a roadside stand and buy a contaminated burrito containing the type of WADA-prohibited substances that can get you banned for years.
In a typical long distance running training program, weeks of hard training are followed by a recovery week (or two) allowing the body to re-energize and avoid stagnating in a running plateau. But, as race day approaches, a runner has to make sure they are in their best form and healthy on their race day.