What should I eat or do for recovery while training hard for a marathon?

I am training for a marathon and every time I start to get into the higher mileage within a couple days I start to get sick. I know it is just the extra strain on my body that causes me to weaken. What are some diet or products to help refuel my body when I train hard? Should I have one week where I focus on mileage and another weak to help in strength training to help break it up?