Once you finish the 30-day Challenge (which basically introduces you to the exercises, how you should be moving your body, how you should be moving the Wiimote, etc.), there are a list of pre-determined exercise sets you can pick to do. I haven't done them yet, but there's a looooong list. Upper body, lower body, sports, cardiovascular, etc. They also vary with regard to time, so some of them take only 15 minutes but some of the more intesense ones can take up to an hour to get through. Or like pp said, you can set up your own, but that seems like too much work to me.
And no, I didn't "successfully" complete the 30-day Challenge. I do other, non-game workouts as well, and it got mad that I didn't do 20 workouts in exactly 30 days, so I didn't reach that goal.

There are other goals you can set, relating to calories burned, number of workouts per number of days, and...crap, there's another one that I can't remember. I thought the goals were kind of lame and didn't really pay much attention to them...