You could tell friends you think you might have a cold coming or something, then if you need to take more rests, they will think it is because you are a bit below par. That is if you don't want to tell anyone about fasting. It is annoying when you have to justify to people what you are doing, because most people don't understand, and think you are nuts lol.
40 days is a long time, hope you make it. I can imagine even if you look fit and well after that amount of time, people won't believe it anyway, even if you told them after the event.