Your meal plan is tailored to give you the best chance to burn fat and make progress. The amount of energy burnt whilst training for a marathon or other endurance events will be far greater than the workouts in your plan. This means you may not be fuelling your body properly, which could impact your progress and performance.
Whilst the plan does provide excellent structure and a nutritionally balanced diet, it might be difficult to incorporate marathon training effectively. For general advice, if completing runs longer than 10km, we would suggest you come off plan that day and change your nutrition. For example, consider having a large meal with a good portion of carbohydrates before the run, some sort of quick energy snack(s) on the go, and a refuel meal post run. It's really important your body has sufficient energy to cover longer distances.