I personally usually workout on the treadmill for about 45 min to an hour and then also do the elliptical machine for 20-30 minutes. Working out at a cardio level for only 30 minutes might help you loose weight but it is not really doing much good for your air or heart endurance. I know this because I am an asthma patient...and I can only do so much when it comes to cardio. However...if what you are doing now is working stick with it!

I usually set the pace at 3.5 and the incline at 1 or 2 on the treadmill and then then lower the incline completely on the elliptical and go for as long as I can. Hope this is helpful to you!