I've been rehabbing a bad toe, but it's finally getting healed enough to use the Nordic Track.
Not my pic or machine.
If you've never used them, or never c-c skied before IRL, it may take you a while to get the rhythms and form down, but by the time you put in 30-45 minutes total, you should be getting good at it. Start for a goal of 5 minute, then 6,7, etc. pretty soon you're doing 45 minutes, having a great cardio workout whild building lean muscle...low stress on the knees as well.
The best part about these things are that you can get them used for dirt cheap, especially here in the South...check Craigslist. The Pro is the lowest model I'd go, but actually the Achiever is well worth waiting to find. it adjusts tension on the skis on the fly with the spin of a knob.