Hiya, just visiting from Slim and Save
Have you thought about the S&S refeed? It's over 7 or 10 days so it would give you time to get used to food again. It's here
http://www.slimandsave.co.uk/refeed.pdf I did it to refeed and had no ill effects, in fact I lost weight on the refeed.
On the day of the race I would say not to worry about carbs as you will be using them so they will not be stored. I would be tempted to have a double portion of porridge with a teaspoon of honey, either with a banana or take a banana in your bumbag for during the race. If you run out of energy during the race it will make it 10 x tougher, and again if your meal hasn't settled it will make it uncomfortable too. By introducing food before the race you can work out how long before the race you should eat.