Cycling ftw. The resistance will do wonders for lower body and core strength building. High intensity interval sprints will get your heart rate up and get you out of breath easily for your cardio and lung capacity. You only get out what you put into it, as with anything. Crank up the resistance and RPM.
All that, and it's very easy on the joints. At least for me it is, I started after I tried running high mileage in too little time after being inactive for years, in $15 shoes no less, and caused ITB overuse. Treadmill and elliptical both agitated it, but I could jump on a bike for hours and it wouldn't bother me.
If you can, do it in a spin class with an instructor who is an actual out door marathon cyclist and you'll get your ass kicked pretty good.
I'm doing 600-800 calorie 1 hour rides 3-4 times a week. HR between 120-180 the entire hour. It's also helped greatly in re-building all the little balancing muscles, restoring oxygen capacity, ligament strengthening, etc, so my running speed and form has improved tremendously now that I've recovered. There have been days where I went into it high energy and my quads and calves were absolutely destroyed the next day from "hill climbs", so the potential for a good workout is there. I find it to be a convenient way to get in your cardio and lower body resistance training at the same time.
The only problem I see with it is your upper body will fall behind very quickly if you aren't doing any upper body exercises.