It's very much a journey. Focus on the journey (on being in the now) rather than the outcome (getting to x hours/days or some other goal). I would also caution against trying to build too much expectation into this fast. I usually do a couple of long fasts (4+ days) a year and always learn a little more about myself.

I've done green juice fasts in the past. I'm sceptical of their effectiveness. The water fast will give plenty of time for your liver & kidneys to clean house.

You will definitely want to get yourself some electrolytes (salts). Magnesium, sodium & potassium are the 3 I prioritize. Search for snake juice recipes if you don't want to pay a premium for salty water. I usually start dosing these at around day 3. I tend to take more than required & assume that my kidneys will filter out the excess. Potassium is good for low blood pressure & light headedness. Magnesium is good for muscle cramps & twitching.