Please be carefull. Please be careful with back pain. It could be something as simple as muscle imbalance, or something worse. When you find the reason you could address the issue in a direct way. Good luck thanks.
Warm up. stretch. I have found that sometimes the back area can tighten up after running. I believe that running at a distance at a steady pace and then resting or sitting down exacerbates lower back pain and tightness. My suggestion would be to stretch the lower back after running and keep moving after a long run and not sit down for awhile after running ie. Walk for a few minutes.