Which exercise? Weight loss weakens back

nxd10

I've lost 20 pounds and added/strengthened/toned muscles. The fat I've lost is mostly in my stomach and bottom. That's great!

The bad news is that I now have a lot of extra space in the my middle and the muscles that strengthen my back are essentially too big to do the job properly. A lot of women have this after they have a baby. I need to strengthen them as fast as possible. If I am not very careful with how and where I sit, I get serious pain in my hip and down my leg, making it impossible to exercise. I teach and have a standing desk, so I'm on my feet all day. I need a strong back.

How? I do home exercises to strengthen my back and abs - crunches, planks, etc. I walk 4+ mph for an hour every day. Other suggestions?