My students are allowed to walk or jog during this part of the class. If they walk I encourage them to walk faster than "window shopping pace", and to pump their arms vigorously. I might mention what a healthy fitness level might be for a child of his age and gender, and motivate the student to reach this standard. I also explain that we do not always reach this standard at the first try, and that consistent hard work is what will help him/her achieve the established goals.
My most used tool in this cases is self-competition (working to improve and "beat" their own previous scores). We do not mention who is the fastest or strongest. I do let students know if they are winners when they manage to improve any of their scores. This way we have a lot of champions and everyone has the chance to excel!
Another useful tool is giving them implements to use while they walk/jog. This can be a soccer ball to be dribbled, a tennis ball to be tossed between partners, a frisbee to pass back and forth, or even tossing the frisbees in different holes located around the cardio circuit for them to walk/jog and toss, retrieve, and continue. Anything to keep them moving happily and not necessarily counting laps, which can get to be quite boring for them.