I have to agree with mo-litia on this one. From my own personal experience, I can tell you that elliptical trainers are easy on the legs, but they do not compare with using the treadmill. I fell into the same trap, and as a result failed my first test as well. I was doing about 45 minutes at a stretch, thinking I would kick some a*s on the test, and it was close, but in the end I flunked. So, I went on the treadmill, and at first had trouble doing even 5 minutes at a stretch, so all that time on the elliptical trainer was pretty much wasted.
Plus, if you're like me, you might need some stress on your joints and ankles to prepare you for the rigours of training when you do get into basic. I found also that when doing the step test, I really felt it in my lower back at the end of the second segement, and running on the treadmill has helped that as well. But, hey, I'm pretty old, and was very out of shape. If you can't run for very long, slow down to a walk for a minute or two and then run again until you can't run no mo'.
Hope this helps -- I saw your post and said "dang, this is exactly my situation." I have my second test in January and hopefully I will squeak in to my reserve training which starts on Jan. 28.
Good luck!