You'll certainly get this in the course of completing a 9 week or cardio program, or you can do this.
Set up a custom workout, high intensity, and choose step aerobics and any other exercise related achievements you may need.
You have to be in perfect step with the trainer or they won't count.
One high intensity set is 178 steps, so you could easily get this achievement in one workout with 6 sets of step aerobics. The achievement will pop at the end of the workout.