Set up a rowing machine screen so that it shows your meter count and your split time at /500m. Then alternate between two minutes medium rowing at 2:20 minutes / 500m and one minute sprint row at 2:00 / 500m. If you are already in excellent cardio shape, feel free to increase the time, distance, and intensity of the rowing and elliptical interval training.
Elliptical Machine Intervals
Set your incline at 75 percent and your tension at 60 percent on a standard elliptical machine. Alternate between two minutes at medium speed and one minute at full sprint speed.