Don't get involved with stuff you're unsure of. Unless you can commit yourself to following through with something all the way, don't commit yourself at all. Too many people are counting on you. Why start something when half of your brain is already coming up with an escape plan? If you don't want to be involved, then don't get involved. People will respect you a lot more if you are just open and up front about your concerns. Flaking out at the last minute isn't going to win you points.