Nutritional requirements for Ultra riding?

Hi; I have got into ultra riding; basically 15hr days at a slow place (IF is around .55-.58)
That works out to around 6500-7000 kj a day. In the past I have just eaten loads ! But know trying to be a bit smarter
Given its low intensity a lot of it will come from fat; so just need to work out the carb requirements. I have never been tested on my fat burning; but assume its pretty good.
For those that have; is there a general rule of thumb for trained cyclists - ie .5 IF is x % fat; this falls to y x at .75 IF, and almost 0% fat above .9?

Many thanks

Eat. Everything.

(My serious reply rather, is to ‘eat balanced’, like a normal person – instead of us bonkers cyclists. The mix of carbs, fats, protein, sweets, etc in ‘real food’ is probably what would provide the most nourishment and mental energy. Just my 2-cents of opinion)