I am on my fifth day of water fasting, and I have just (literally right now) completed a five mile run, which took about thirty minutes.
I've previously done a forty day fast and continued to work as normal, although I will concede that it wasn't easy after day thirty or so.
So my advice (for what it's worth) would be to refuse to accept the weakness and you might just be surprised at what you can achieve. Shelton states "if you think you're weak you are" and I could not agree more.