So now that I'm back walking, I notice a very slight, dull lower back pain. My back doesn't like the jarring. So the last two walks, I've increased the time (30 minutes out, 30 minutes back) but am walking slower, up steep hills. I'm hitting a great HR (130) going up the slope(s) and on the way downhill I maintain 115 (remember I'm in my 60s so that's OK). But with the slower pace, no back pain. Meanwhile, I'm starting crunches which historically stop the back pain - I don't know why I stopped.