Logic says it's mostly fat.. after all: your glycogen is mostly gone.
However: the less carbs you eat, the more carbs are coming from protein. Day by day about 100grams of carbs are derived from protein, even when you eat zilch, nada carbs.

If you don't eat the protein, it'll be derived from muscle. That's what happened to me when I reeked after ammonia during exercise this summer.

Also: 800 kcals/hr doesn't sound very likely.. 500 kcals seem more likely. at least that's what I burn per hour when I do interval cardio for 25 mins on a Johnson elliptical. Other brands go way higher, while resistance feel lower.. go figure.

So... it would not be around 800 but more like 500-600 .
and about 10-20% would be coming from carbs I guess.
More like :500-50= 450 kcals = 50 grams of fat?