I’m not a nutritionist. However, I sometimes play one on the internet 
That said, this is very individual. Common recommendations are 60-90g carbohydrate per hour. I believe this is the limit of what the body can actually absorb/use per hour. 1g carb = 4 cal Thus, 240-360cal per hour.
You could do this with solids, liquids, or some combination of the two. You will need to experiment in training with your choices to make sure that you can literally “stomach” your plan.
For example, I do 1 bottle of carb mix and 1 gel per hour which puts me in the 60-90g range. I did this yesterday for a ride over 6hrs w/~3300m of climbing. I never felt hungry. I never cramped. Energy was great. Had it been hotter, I probably would have taken on some additional water.
Again, make some choices, and test them in your training. You do not want to find out on race day that your stomach does not agree w/your plan.
edit: I missed the part about 21-22hrs. Wow! In this kind of event, do people stop for meals?