Someone on Twitter calculated about 55,000 calories burned. Not sure if he had things to consume while running - but 8 - 10 lbs of fat burned in one day is wild to think about.
So calculating the net burn would be quite difficult. But the gross burn was probably about five pounds/2.25Kg.
Oh, he had things to consume. I ran 16 miles on hilly terrain on Saturday, and just for that I brought about 500 calories with me (not all consumed, some was "just in case").
It has been said that your liver stores about 2000 calories of glycogen. Divide by ~100 calories/mile, and now you know why marathoners hit "the wall" at about 20 miles if they don't chomp on some calories along the way.