I need 60g carbohydrate per hour to keep me going over long distance. Tomorrow I've planned an 80km bike ride and a 20km run - at training pace that should all take about 5 hours.
On the bike I'll take 2 bagels each with 30g of Nutella in (yum) plus a 500ml bottle with one sachet of High5 powder in. High5 to be sipped throughout the ride, and a quarter of a bagel to eat every 15 minutes or so. I'll take water in the other bottle. Returning home for a quick transition, my run belt will be ready with another bottle with High5 powder in, and 4 gels - one as soon as I set off, and at each 30 mins afterwards.
284.4g of carbohydrate over 5 hours gives 56.9g per hour. This is a touch on the low side, I'd aim to be 10g per hour above this for a very long day, but should be enough to know if the bagels are OK.
To try and make sure I'm not "cheating" by having a big breakfast that would mask the effect of any under-fuelling I'm going to keep breakfast light. A cup of tea and a single piece of toast with no butter and 20g peanut butter should do the job.
One difference between this experiment and race day would be I'd probably go with white bagels on race day to keep the fibre level down.
It's important to train and test every aspect of the race - there's more to this lark than just swimming, biking, and running!