Set your monitor for a variable interval workout as follows, with no rest between intervals: 500m, 500m, 400m, 400m, 300m, 300m, 200m, 200m, 100m, 100m, 200m, 200m, 300m, 300m, 400m, 400m, 500m, 500m. Ski the first 500m at a moderate pace, then 500m at an intense pace, then 400m moderate, then 400m intense, and so on down to 100 meters, then work your way back up to 500 meters in the same manner.