You will find that Lent is longer than 40 days, so dial that in as you see fit. You could fast the full 45 days or, for example, fast on a 6/1 schedule…Six days fasting w/one day of eating, in light of the fact that every Sunday is traditionally a non-fasting day in the liturgical calendar, due to it’s status as a perpetual feast day in rememberance of Christ’s resurection. Commit to eat keto or proximate-to-zero carb on those Sundays and you should be able to slip right back into a water fast with no hiccups, especially the further you get into the extended fast.