Using a constructivist model, how can Rebecca structure the year so that her students will be motivated to improve their times when they run?

Rebecca is a high school P.E. teacher at a large urban high school near Washington D.C. As part of the physical education standards, Rebecca must time her students periodically in the mile run. Within her classes, she has students of all ability levels and so as she begins to plan out her year she is struck by the challenge of motivating each student to improve in the mile. She does not want any students to feel alienated and instead she wants everyone to feel motivated to improve. The mile run will be tested three times, once at the very beginning of school, again in the middle of the year, and once more just before school lets out for summer.
Using a constructivist model, how can Rebecca structure the year so that her students will be motivated to improve their times when they run?
What can she do to ensure that students at every ability level are challenged at their skill level but also pushed to improve?