Two choices: 1. In the past, the morning of the day before the fast I cut back to a very easy run (both pace and distance). Then the evening after breaking the fast I do the same (the day after the fast is also an easier run); or 2. Don't fast, which is the option that I've taken the past few years (honestly since I had children I cannot fast anymore, it's so much more difficult when I'm giving them breakfast and making them snacks and lunch).