The "worse challenges" you have seen are not realistic. I was part of a challenge where the person who started the challenge wanted to lose 60 pounds or so in 3 months. That person never weighed in once during the challenge.
Some challenges are even pro-ana, ridiculously unhealthy and unsafe diets for and by anorexics who care more about losing every last gram of fat than their health.
Right now I'm on a "lose 12 pounds for Christmas" challenge. Not a single person is going to come close to achieving that.
I want to be a size 8, too. I have lost 46 pounds since the end of May. I am currently a size 12, and estimate it will take me another 3 months to get to an 8.
I ordered some calipers so my boyfriend and I can measure each other's body fat, because that's a more sensible way to set a goal.
But you know something? People have been telling me how "hot" I was since I've been a 16, and you would be a "hot" size 16 or 14 or 12, too. They sell great clothes at that size. This scientifically researched book
is written for people who are going to go on a crash diet no matter what. It is a protein-sparing fast with exercise that should help you cause as little damage as physically possible in the process.