Interval training could be what you are looking for if you are at a plateau in your workouts. Interval training is basically exercise which consists of activity at high intensity for a period of time, followed by low intensity exercise for a period of time. These 'sets' are repeated. You can do interval training in many different ways. For this type of training you can use a bike, elliptical machine, treadmill or you can even walk. The point is to just do a level of high intensity and then rest for the same period, or if still not recovered, take a little longer on the rest period. Here is an example of interval training I was doing today on the spin bike.
5 minute warm up at a medium tension.
1 minute at a medium to heavier tension as fast as possible.
1 minute rest at a light tension These two steps are repeated for 30 minutes, then...
5 minutes more at a medium tension then back to the interval training as described above for 5 minutes,then...
3 minutes cool down. To me, this is a pretty intense cardio session. I know some of you reading this can do more, and that is great. However, if you are not able, go at your own pace and pick your own levels of intensity and rest periods.