Carb loading is a way of boosting endurance when exercising for longer than two hours. As Tim will be running for at least 4 hours, he thought it would be useful.
There are conflicting views about carb loading. This article gives the science behind it: basically, the body can only store a limited amount of carbohydrate, so carbohydrate loading trains the body to store the carbohydrate in the muscles and liver.
In the first part of the week, carbs are limited, and protein is high. As the week progresses, more and more of the meals are made up of carbohydrates, up to 75% in the final three days before the race. This is supposed to give optimum performance.
So far, we've had steak, chicken and tonight its salmon. I've got to say I'm not disappointed. I think the best part of it is that the foods that are recommended are really healthy, and the psychological effects are huge - even though the actual science behind carb loading is debated, Tim knows he is giving his body the best possible chance to do well.