Not sure what positions you can get into after the surgery but there's a fab way to activate your back muscles either laying on your stomach on the ground, on a bench, or (my favorite) on an exercise/balance ball. It may look simple and easy but if you haven't done these before, you'll sure feel them. They're a great way to wake up back muscles in preparation for heavier lifting you plan to do weeks and months later. There's a bunch of related exercises in the video below.http://www.ehow.com/video_4957687_lower ... 9a0f4d61e1
This one might not be for you at the moment but I recommend it in the future because it strengthens the heck out of the glutes simultaneously with the lower back: Lay on the ball so that it is right under your lower abs. Roll forward slightly to stabilize yourself so that your arms are like two front legs on a chair and the ball is the back leg. Then, using this balanced platform, raise and lower both your legs behind you as far up as it feels comfortable to do. Move slowly and deliberately, keeping your legs perfectly straight, and do not bounce the ball as you move. Squeeze your glutes at the top. This one helps works kinks out of a back that sits all day and it's a great move to do in the transition from your desk to running in the evening. It also gets your butt ready for squats, literally and figuratively.
If the balance ball puts too much pressure on your abdomen, you can do truncated variations of most of the above from a face down laying position on the floor.
You're on a good path, smart and strong.