Beginning a longer water fast

First, a little bit about me. I'm a 28 year old male, 5'10", 88kg. I've come down from about 120kg slowly and steadily over the past 4 years, and over the past 6 months I have been doing weight training. I'm in good enough shape to successfully finish half marathons in a decent time.

Last weekend I tried a one-day water fast, and on the whole it wasn't too difficult. After I finished, I felt a little better overall, but it was hard to describe in what way I felt better because I'd never had quite that feeling before. Anyway, this morning, I was doing some more research about fasts and I became very excited about doing a longer one. I'm thinking 10 days to 3 weeks. I have a few questions:

I donated a kidney about six years ago, and my remaining one is functioning well. Will a fast that long be too stressful on it?

I've heard it recommended that you start out with a short juice fast before you get into water fasting. Is that because plunging into a three week Water Fast is too shocking to the body, or just because it's a bigger mental challenge?

Is it safe to live your life as you usually do (in my case with maybe less or no weight training), i.e. working 5 days a week, driving a car, etc.?

I live in Japan and have no idea where to find a knowledgeable person to supervise my fast personally. Is this a big deal?

That's all I can think of for now. I'm sure there will be more questions later. Thanks for your time!