First, ive done a lot of research on this subject and was very interested in this type of training, but I am by no way an expert, so read with caution (and welcome to discussion)
Ive competed in Marathons, Tris and Xterras.
First, what are you familar with now? Why are you trying to change it?
From here on out, I wont be able to produce studies at the moment, but if you look for it online, youll find it. A lot of this is from TriFuel and other similar sites.
Im with Mike in saying that getting your body to burn fat during your long runs is a product of well, long running. This happens because your body can only absob something like 300-400 calories an hour, but will obviously burn well in excess of this every hour of that run. Your body then learns to burn fat and use that as fuel when that is burned out.
I am familar with low carb diets and their principles, but I beleive those are more suitable for power lifting applications than in this case. I may be wrong, but when I was on a low carb diet while training, my long runs suffered.
Whats your training scheduel look like now too?
Heres a way to put it into perspective though. Lets say your 50 mile race takes ya about... 7 hours to finish? Thats a lot of fuel and liquids youll need to replace during that run. What do you plan on eating? Fat? Probably more likely fruits/nuts and or gels/gus or some sort. I think we can all agree that sugar burns much faster than fat, and thats probably what youll want during a race. Youll never be able to put in enough fuel into those muscle 'carb loading' only 2 days before to last you through anything over 3 hours at race effort.
Carbs are better for long distance run fueling ( >4 hours) IMO
Besides, the training involved in distance running burns so many calories, just eat everything in sight and taper/clean up towards the end.