My partners friend is a personal trainer and I asked him what to do, I asked if sit-ups would help and the look of horror on his face told me no. He said that after any birth the muscles have separated and if you do crunches you are only going to make it worse and cause serious health issues in the long run. Anything that causes you to flex your spine is a no no. He didn't say for how long but he did say you need to strengthen the core muscles first. Pilates and Yoga and walking is a good start he said, just don't do any of the spine flexing moves like the hundreds. Now to just find the motivation and time.