I found this, it's for a mile of swimming, but you can easily adapt it to your physical abilities. I went swimming for my cross-training the other day (also with a half-hour elliptical workout) and let me tell you... it was a pretty great workout. I would agree though, that doing the backstroke doesn't feel that demanding. I usually do a length and back of that if I need a quick break and then go back to something more challenging.
Try using a flutter board and just using your legs for a few lengths or a buoy and just pulling with your arms.