Hey, runners! I desperately need some advice! I run approximately 2 times a week, and can go about 3-4 miles. My problem is that toward the end of my run, my lower back really starts tightening up. I think I could run farther, because I'm not really that tired, but I have pain in my back. If I stop and stretch it out, it's hard for me to get started again. Do you think I need to strengthen my lower back, and if so, what exercises should I use? My husband thinks I need to strengthen my abs and core, but I have really been working on that, and the back tightening hasn't improved. Any suggestions?