Going back many years ago, I began having lower-back pain. Not a lot of pain but enough to cause me concern. It happened that at the same time, Runner's World had an article about a way of doing situps that put less stress on ones back than the traditional army way (my stretching page describes the way taught by the RW article and also the stretches I do for my lower back). I began doing those sit-ups and some stretches for my lower back. It took about a month, but the lower back pain went away and has never come back. I had been running for several years before the lower back pain came. This was a few years before I ran my four marathons.
When I first moved to Utah 14 years ago, I did run but not a lot of miles, and I was careless in not doing my stretches regularly. My back muscles became so weak that I had a hard time getting up off the floor after finishing my sit-ups. I had to turn on my side and rise to my knees and then stand up. I realized what was happening to my back, and I become more regular in doing my sit-ups and back stretches. As before, it took some time, several months, to get a strong back. Now, even with my old body, I have no lower back pain and I can rise up from the floor and stand on my feet.