Cybex Arc Elliptical Trainer, Level 5 Hill: 45 minutes steady state. This machine is awesome. It not only adjusts resistance but also stride length and height depending on the hill. Most of my time was spent at a BPM above 168.
If you haven't yet, give Kris' Under the Bar and give the entry "Hope" a read. There is a new entry about his back problem that is so complete it may help you prevent future back problems of your own.