I've fasted for 2-3 day periods where I was generally as active as I'd normally be, including doing a few demanding 60 and 90 minute yoga classes during that time. The only thing I really noticed was that if I exerted myself above a certain level, such as walking at too fast of a pace, then I'd very quickly feel tired - all it took was reducing my walking pace and I felt fine again; I presume this is related to the different rates of energy being made available.