Most runners eat a high carb diet 24/7. Combine that with high mileage and it is the fast track to a SKINNY FAT appearance, CVD and diabetes.
Needless to say, it is also counterproductive for Marathon performance.
What you need to do is fuel wisely.
Do a Long slow jog (like 20 miles of SLOGGING): only eat fat/protein before and don't fuel during the run (except water).
Have a hard workout planned, e.g. CROSSFIT STYLE SPRINT REPEATS, like Al Salazar and Gallen Rup are incorporating? Consume some easily digestible carbs right before the workout.
The day before the Marathon you can eat some carbs, too (e.g. sweet potatoes). Just stay away from grains.
Many professional athletes have changed their minds over the past few years and are incorporating a PALEO based fueling approach into their Marathon and Marathon TRAINING.