The elliptical is going to be a controlled motion different from regular runnng but will decrease impact on your joints. The impact on the joints from running on a treadmill or outside on ahard surface can be up to 7 times a person's bodyweight. The best cardio/fat burning strategy would be to use interval training. Higher intesity, lower volume, short bouts followed by recovery. You can use work:rest ratio or a heart rate monitor. Hope this helps.