Good. I recommend you take it in 7 week stages. At the end of 7 days evaluate how you feel and decide whether to continue. Fasting is a muscle you have to strengthen.
How I built up to my 40-day: 14 days breaking my refined carbohydrate addiction, 11 days of increasingly restrictive Intermittent fasting (16:8, 18:6, 20:4 & OMAD) and then 4, 5 and finally a 7-day.
Here’s how I broke up my 40-day