I did a waterfast for 9 days, a few years ago. Like people will tell you, you just gotta make it past day 3 or 4 or so (I think it was day 3, if I remember correctly). After that, it'll get easier.
Probably best to take it easy, stay at home, sit on the couch watching TV or something like that. It's not good to exert too much energy during a fast like that, I've heard. And of course please be careful when starting eating again. Apparently people have died from doing that process wrong.