"Dr. Sara Brown, a sports physician in Lincoln Park, said fasting runners should sit out the marathon, adding that long runs after fasting can be dangerous."
http://online.wsj.com/article/SB123395183452158089.html
As I understand this, an observant runner would be fasting from Friday night until Saturday night, be able to eat Saturday night, and then would run Sunday morning. The fasting may be sub-optimal, but dangerous? I don't think so.