My views on dieting while training...
You will be a better marathoner if you weigh less.
Starvation diets while training are foolish.

But unless you're running way over 60mpw, you should eat when hungry, but not to excess...and avoid fatty foods.
My PR was at a higher weight than some others, but only 5 pounds(and the course and weather were perfect). I was using training as an excuse to eat like a pig--the running had become a crutch.

But your joints will receive less of a jolt if there is less weight on your body. I just cut out a few foods and snacking and dropped 15 pounds(treating an injury) with the same mileage. I still eat a lot.

But again, no starvation...<1000 calories...we need the fuel.