I'm the kind of person who does most small workouts somewhere over the course of the day so that's my preference. You probably have to experiment a little, see what works best for you.
If you use the Fix It, Lower Back Workout, and the Glutes of Steel Challenge I would do the first two whenever you feel the need for a bit of movement. You can also spread the sets of the two workouts out, for example, one set of Fix It every hour or so. If you feel like the Lower Back Workout is too much in one go, just do one exercise, the next one 30 minutes later, or whatever works for you. I found that just the exercises themselves already gave some relief, so they worked (for me) outside the workout, too.
I liked to do the Glutes of Steel challenge at that one point of the day when I felt like I was already sitting for a long time and my butt felt like a pancake. I would recommend during the bridges to not just lift your hips from the ground, but "push" with your feet through the ground. And when you're in the bridge position, push and prod your butt a little, see if you feel the muscles working. Depending on how lazy those buggers are, you need a couple days worth of exercise until they start doing their job. Lying side leg raises, Jane Fonda style, are also excellent for the glutes, especially because you can feel them up while doing them.
If you're fine with doing squats, I found that doing a couple whenever I went to the kitchen or bathroom was time well spent, too. There's a lot of small things you can do now and then. While brushing teeth, waiting for the coffee, on the phone, whatever, like calf raises, side leg raises, back leg raises, every bit helps.
Good luck, hope you can successfully combat your back pain.