I also find it (prev) useful to dilute water and Coke 50/50 (as it brought the Cho concentration down to ~6%) and to keep my fluid intake < 600 ml / h, i.e. ~150 ml every 15 min's or so. BUT as I got slower (and more bored!?) near the end of an ultra, I exceeded this by quite a margin in the past, causing the loop of - feeling tired - drink Coke - feeling MORE tired (because of the CHo), drink MORE Coke etc. I just didn't realise it at that stage. At the moment I am in an experimental phase re this, but I have not run far enough for proper testing since I started with banting. (Only up to 21's - this will change soon though) So to be honest I am not yet sure how I will approach a mar+, but we will see.
