For me, i would say its 70%-80% mental. It doesn’t matter if you’re really fit, if you are mentally fried at half way, then you’re race will probably be over.
Get a few good helpers that will be there for you, cheering you on and dealing with any problems you’ll have.
For training, i would say you should concentrate on Muscular Endurance and forget about 6-8hr runs.
There are people who do 12-24hr time-trails, who only do short hard training sessions before the races, usually around 8-15hrs a week.
I tend to do things differently from other people, if i’m doing a race like that, i would just go as hard as i can and try and keep it up!!
BUT, everyone is different and that might not work for you, but you never know until you try.
Good luck