Preparation is all about routine for me, from experience I know what I can or cannot eat on race day. This experience is built from trial and error. Sometimes it will go wrong!
On the day I have a very simple pre-race nutrition plan keeping breakfast to my standard pre long run staple. This normally consists of cereal (Weetabix) and coffee - making it simple I am travelling abroad.
For London, on the way to the start and up to one hour before I will drink one bottle of fluids consisting of Nuun hydration tablets. I try and keep my fluid consumption minimal as this allows my bladder to empty before the race starts. Again in the past I have over drank and I promise you don't want to be needing to relieve yourself once the gun sounds on your target race.
Finally, before the start I will consume one gel, this is to provide a final boost and keeps my energy levels high as soon as I am underway rather than taking my first gel five miles into the race. Currently GU gels are my gel of choice and I quite like the coffee flavour gels for pre race.
Being a creature of habit for my road races and quicker paced races I just carry gels with me. Over the marathon distance I start to add chews and bars. For the marathon I carry five gels, this is a total of six gels including the pre-race gel.
I stick with one gel every five miles for the first four gels up to mile 20, following that I have the 5th spare gel that is taken on feel. This is the only pre race fuel excluding a few sips of water from aid stations.
Again from experience it's very important for me to get some food into my body as soon as possible after a race, I tend to consume a recovery shake and some sort of nutrition bar before attempting the first pint after four months abstaining, also salt and vinegar crisps are a winner!
It's important to ensure you can stomach your nutrition, I always try consuming the same gels I will on race day in a long run in the weeks building up to race day. However, I would not recommend taking too many during this period as you want the benefit on race day and not for your body to become reliant on them.