Hi there! I just finished a 72 hour water fast last week. I used to fast a lot several years ago for spiritual reasons, but was not wise in how I entered or ended it. This time I didn’t even try a longer fast until I was accustomed to daily intermittent fasting (only eating from about 12n-7pm). That made it easier. Since I’m still a praying person, I asked for lots of grace, too!
I did learn to wean from coffee ahead of time or expect headaches and lethargy – ugh. My back-up plan for salt was pickle brine (from true fermented pickles, not just vinegar). I use that sometimes when I feel a need for it. But I didn’t need anything more than water – yay!
Day 2 was hardest as far as “wanting” food, not really needing it. By day 3, all was well. I ended up having to break the fast because of family plans. But the experience reset my keto progress that went by the wayside during the holidays… 
Keep up the good work! It’s worth it in the long run. But it is easier if you are on the road to fat adaptation already. Learned that lesson!