I would check out the video on the “Lunge Archetype” on the getting started page.
As a runner you want to make sure you have adequate hip extension. If your tight in the front of your hip there is a tendency to extend in the low back which can lead to discomfort in the low back. So a great place to start is opening up the front of the hips. If you find your lacking hip extension here is a mobility routine to get you started.