I had back surgery (microdiscectomy) for a ruptured disc between L4/S1 on July 31, 2002 and thankfully, my wife is a Pilates instructor. I went through 6 weeks of traditional PT after the surgery with some limited success. The back pain was mostly gone, but it still felt somewhat unstable in my lower back.
I then started doing private Pilates lessons twice a week. That made a HUGE difference. The rectus abdominus is only one of many, many muscles that contribute to supporting the spine. No pun intended, but the "core" of Pilates is core stabilization. If you want nice flat abs and a strong back, I would very highly recommend finding a competent Pilates instructor in your area.
Good luck with the back injury, they really suck!