OTF is dynamic and matches the fitness level of the person engaging in the work out. The best way I can explain it is, when I started I could only walk during the treadmill portion of class. Walking at different inclines was enough to elevate my heart rate through all the zones. The last challenge I did (about a year later) I ran a 6:54 mile. Now when I'm on the treadmill, I have to run and run hard! The workouts adapt as you get in better shape, so no matter what your fitness level, you will get an appropriate challenge. I love it!
You can push yourself however hard (or not hard!) you prefer. I also really loved the instructor, Lou. He did a good job checking in to make sure the newbies were comfortable and was encouraging but not overbearing.