I don't recall what Fuhrman said in his book, but I was monitored by a health professional on my second fast and he said, "Drink as your thirst dictates, but no less than a quart a day, and definitely no more than a gallon as you can quickly deplete your mineral stores."
Concerning exercise, he said that relaxed walking was great - but ONLY when you feel up to it, and desirous of it. If you're having a weak and tired day, relax and spend more time in bed, or resting.
You must consider your HEALTH when fasting, as a change in lifestyle. If this is for weight-loss only, you can quickly gain back the weight when you break the fast and end up worse than when you started.
I know; I've been there.
Call it the voice of reason, the school of hard-knocks, but I know because I LOVE food. It is my 'drug'...
...and I've been through a few fasts before I finally got it into my head that water-fasting is one of the best things to do...
...do it for the right reasons.
Weight-loss should be a "perk" of fasting - a "bonus" - not the main goal.