Back muscles can get sore from hours of pounding if you're a hard lander, or if you tend to run with a tense upper body. Runners who are new to ultra running and want to do it for many years should consider some ab and back exercises to strengthen those areas. What works for me are exercises included in my weight lifting routines. I use relatively light weights but do the exercises 2-3 times per week. In the 3-4 years that I've been doing these, no back or ab problems.
Some people get really tense in the upper body when they run.
It's no big deal in a 10K race, but you can't run 50 that way without getting really sore. Be aware of your body while you run and learn to relax if you get tense. Others on the list probably have their own favorite ways of strengthening abs and back muscles. One other muscle set that can fatigue in ultras is the one that is used in breathing. We take breathing for granted but don't realize that many hours of breathing at a faster rate can tire those muscles.