So, easy runs are for aerobic gain. Therefore doing the elliptical you're practically gaining all the benefits you would from doing an easy run with equivalent heart rate without the impact. However, the question I need answered is: If you can raise your heart rate to about that of an easy run and hold it there, are you gaining aerobic fitness the same way you are while easy running? Such as using an "arm bike" (hand pedal machine). Or is the factor of using your legs important?