Have you done a long fast before? 31 days is pretty long if you don't have prior experience.
A good rule of thumb is that you should never do a fast more than twice as long as the longest one you did before. Hence, if you are doing a 31-day fast, you should have done a 16-day at some prior time, which itself should have been preceded by an 8-day.
You may want to break it at day 16-20, refeed on vegetable juices, and then stay on fruits and vegetables for 3-4 weeks, and then fast again...