When I talked with a CM at WL, he thought I was a bit crazy but said it was not against any rules to exercise on unlit paths at night or to use them to travel from one place to the other. There is a path from the WL to FW that is unlit which prompted my question.
Due to my work schedule, I only go out after dark or before sunrise and I only take a light to make myself more visible to traffic. Night really isn't that dark, especially in areas without trees. I also wear reflective clothing to be seen and so that people know that I am out exercising and not robbing houses. I normally follow the same guidelines while at WDW just so I don't creep out other tourists and security.
I've been known to do laps around Hourglass Lake at 2AM before and have never been bothered by security. They probably have figured out that there are people like me who have a genetic flaw that requires them to run daily even on vacation. I like to get in at least a few miles to relax every day and the only time I can get away from my family is in the late night or early morning when the sun hasn't come out yet.