The elliptical can be tough initially because you are using different muscles. However, in my experience the elliptical is a machine of diminishing returns. The more you use it the more acclimated your body gets and the less effective it becomes. The best way to avoid this is to keep an eye on your heart rate to make sure it stays elevated at optimum levels while you are on it. Also, the elliptical is fantastic for knee rehab and a great low-impact cardio workout. Just remember, sweating is not an indicator of fat burning or muscle building. For me, the general rule is, if my heart rate stays up then I'm burning calories and if I'm sore the next day then I'm building muscle. Good luck!
Gaither