Fruits are cleansers. Vegetables contain vitamins, minerals, trace elements and enzymes essential for rebuilding the body, and easily assimilated directly into the bloodstream without putting a strain on the digestive system..
Eventually the body will begin to consume muscle during a fast, but muscle is far down the list while juice fasting. From the excellent book, How to Keep Slim, Healthy and Young with Juice Fasting by the legendary Paavo Airola:
"During a prolonged fast (after the first three days), your body will live on its own substance. When it is deprived of the needed nutrition, particularly of the proteins and fats, your body will burn and digest its own tissues by the process of autolysis, or self-digestion. But your body will not do it indiscriminately! In its wisdom -- and her lies the secret of the extraordinary effectiveness of fasting as a curative and rejuvenating therapy -- your body will first decompose and burn those cells and tissues which are diseased, damaged, aged or dead. In fasting, your body feeds itself on the most impure and inferior materials, such as dead cells and morbid accumulations, tumors, abcesses, damaged tissues, fat deposts, etc. ... These dead cells and inferior tissues are consumed and utilized first. The essential tissues and vital organs, the glands, the nervous system and the brain are spared."
Water is another thing, as the process is speeded up considerably. While fasting on water you need to rest, physically and mentally. The opposite is true while fasting on juices. Having done both, I see no reason to go through the depletion of a Water Fast unless a person is called to do so for spiritual reasons. Juices, or the mono-juice fast known as the Master Cleanse, are excellent ways to cleanse, detox and heal the body while continuing regular activities. Of the two, juices are by far the more supportive nutritionally, though the Master-Cleanse is definitely convenient and effective for the short haul (up to 20 days or so).
Again (and again), for anyone undertaking a long (over 20 day) fast, I highly recommend, along with virtually every fasting expert I have ever spoken to or read, that you work with a natural health professional with fasting experience. There is no way to describe the support I have received from my excellent naturopath during long fasts, without whom I would have folded a fast, probably breaking badly, thus losing all the benefits I had worked so hard for.
As far as exercise... I continued my regular gym routine every fasting day barring crises (there were a couple over the 60 day haul). I do 30-40 minutes on the treadmill, walking hills, then 13 different weight machines. I moderated the workout depending on how I was feeling that day. I also drank 1 T. BLACKSTRAP molasses dissolved in hot water prior to working out for extra energy, and had juice waiting afterward. I am a 52 year old woman, not a jock by any means, but was able to keep up this routine five days a week, plus my normal activities, again, barring crises.