Lots, but make sure it's all familiar food.
Increase the proportion of carbs to about 70% at the expense of fat rather than protein. So pasta with tomato sauces, homemade deep-pan pizza going light on the cheese, spuds, lean meat and veg, bananas, malt loaf, fig rolls, dried fruit, bread of whatever sort you like, breakfast cereals...
But don't get too worried about diet - it's only 13 miles and your legs can easily store enough fuel for 13 miles as long as you're eating enough overall and haev done a spot of training. The most important thing is not to eat anything on the night before, or on race morning, that might upset your guts.
