Once I fasted for 3 days, only drinking water. By day two I was pretty weak, afterwards I recovered pretty quickly. The point being, it shouldn't be to difficult to do if you aren't going to need to be too active.
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.
After seeing this article the first time, I decided to try the 3 day fast as well. I too continued my normal morning 3 mile runs and noticed that if I couldn't push the pace for very long. I'm just an anecdote, but my immune system seems to be stronger this winter. With two little ones who've been sick a couple of times and a wife that had the flu, I've been able to get by with just minimal congestion.