Back pain is a bummer... I hope you'll get better soon.
I tend to have back pain too and walking usually helps. If I do it right: not too slow, not too fast, minding my posture.
I also did physical therapy that involved a lot of ab work, but avoiding some exercises that put too much pressure on the back, like the sit-ups. I always modify when there are sit-ups in a Darebee workout and I take care with the leg raises as well.