I do a 72 hour fast the first four days of every month. I recommend a blood glucose tester, a blood ketone tester and MCT oil. I like coffee with three tablespoons of MCT as an energy booster, decaf when it’s close to bedtime. The Medium Chain Triglycerides are absorbed directly into the blood and do not initiate a glucose spike or an insulin response. They also trigger leptin, the hormone that creates satiety. If your body is not fat adapted MCT can cause diarrhea, be sure to drink plentiful water. The testers, each about $40.00 allow you to see how your blood chemistry is doing and allow you to troubleshoot if you start feeling unwell. A small amount of shakiness and muscle fatigue is normal for me, I test to make sure blood glucose levels are above 80 mg/dl, typically the testers are up to 10% +/- so this gives a margin for safety. At about 36 hours your ketone levels will rise dramatically. This is the beginning of autophagy, and the switching to free fatty acid metabolism for your body’s primary fuel rather than glucose and glycogen. 
The benefit I notice the most is a mental and sensory acuteness that is unique to fasting for me, sometimes I feel like continuing because I feel so good, but it’s best not to go into keto acidosis, the process of metabolizing your own muscle tissue for fuel, not only will you lose muscle, but it is a last effort survival mechanism for your body to avoid organ failure with long term irreversible health consequences. It’s important to listen to your body, if you feel unwell it’s best to gently stop the fast. Drink 1/4 cup of fruit juice to bring blood glucose up, then half an avocado. Wait an hour and increase food consumption eating easily digestible food, keeping in mind that the capacity of your stomach will have decreased substantially. I wish you the best of experiences with prolonged fasting. I think it truly is a superpower.