Fasting is the safest, easiest, quickest and likely the most permanent way to lose weight, cure diabetes as well as a 1000 other ailments. In fact its a way to even cure malabsorption and emaciation. Polar opposites if you ask me.
Its like saying you're so skinny, stop eating a few weeks and you'd absorb all the food better.
The 30cc of thorazine cure.
Anyway being more serious for a moment.

This is the approximate math (Dr Fung intensive dietary management.com has all this info and more)
4 days of increased metabolic burn.
9 more days of slow return to the base metabolic rate.
Muscle is only likely eaten till you get into ketosis - first 2-3 days.
You can help your muscle preservation a good bit by eating a lot of fat the 2-3 days before starting the fast. Eat 3000+ calories of fat. You wont be depositing much of that, essentially you'd be excreting most of it, but your body is used to burning lots and lots of fat. Then it will happily target your fat stores and live off it for months if needed.
You liver can easily make carbs and protein from fat reserves. Just make it do that first with dietary fat and then to body fat.
Exercise can preserve muscle but if you tear or damage anything it will heal a lot slower. So keep it light and steady and low impact. You don't need to do that before the fast. You need to eat fat and lots of it. 3/4 - 1 lb butter a day for 3 days will nicely drop you into ketosis and there on out its smooth sailing till you're in your target weight. BTW I'd shoot for a 10-12% body fat, and staying there a few months will give you a great chance to maintain it for the rest of your life by resetting insulin and leptin levels to there.
Thanks.
Srinath.

Your body will not eat your muscle till all reserves are gone, but by then you will feel the true hunger.

It seems to me you have never fasted. Please note that you will experience headaches, cramps, foul mouth, sleeplessness and other symptoms of detoxing. Also nothing but water goes in your mouth, no supplements or even toothpaste.

Best wishes.

Archus

Hi,
I would love to be your fasting buddy... if you want one. :)

I've been thinking about it trying Water Fasting for a while now, but have been a little reluctant because I have a medium level physically demanding job (hope it makes sense, English is not my first language) and I don't have the possibility to take time off work.

My main goal for fasting is weight loss, any other positive changes are just a bonus.

My plan was to just go for it and start tomorrow.