For runners, sore back is often a reflection of tight hamstrings and hip flexors, and weaker core. I do hip flexor strength exercises https://go.wja.is/medium-deep-bodyweight-squat plus various glute bridges. Then warming up include high knee pull to chest on toes, and hamstring mobility, and after running, in your warm-down hold that high knee pull as a stretch plus several hamstring stretches. Sitting too long exacerbates the hamstring and hip flexor tightness so do the 25-minute break thing, stretch and take a walk. Good luck.