Here is a link to a summary, I couldn't find the actual research articles I was thinking of though.
http://www.pponline.co.uk/encyc/sports-nutrition-should-athl...
It is fair to point out (which the article does as well) that the athletes were running at 75% of V02 max, which is lower than when racing in cycling. I ride a lot, and anecdotally I had a cycling teammate that changed his diet to consist of primarily fats for 6 weeks. The first few weeks his body was adjusting and his endurance and power dropped dramatically as you would expect. After 4 weeks, though, he had come back and had a really strong season.
The way I understand it is that as intensity increases it becomes more difficult for the body to convert fat for a large source of fuel, and glycogen becomes the largest contributor to energy. I know the feeling of a "bonk", when glycogen depletes, is a really strange one and all fuel is coming from fat at that point and intensity has to drop by quite a bit in order to continue.
Our bodies are pretty amazing
There's only one thing that gets you out of it quickly - and that's a shovel full of carbs.