I think 20 minutes of good mornings would put me down for months. Even the tiniest fatigue-induced form error is going to add up over the course of 20 minutes, and my low back can't take that.
That's why personally I'm going to stick to step-ups and if possible cardio machines. No sense turning my aerobic work into a chance to screw up my form and then screw up my back. But if your body can take it, don't let me stop you.