I'm 49 years old. 5'7" 158-160, depending on time of day, etc.
I'm right at the border of "barely overweight/high end of healthy weight".
I've had 4 C-sections, spread over 13 years, the last one 17 years ago.

Thirty years ago, I was an athlete in peak physical condition. I had two of those C-sections four years apart. Then I became a runner rather than a gymnast/strength trainer.

Then I became a model-thin slip of a girl. I lost a lot of my musculature, but not nearly all of it. Basically I went from a 36-24-36 to a miniscule 34-23-34. My weight dropped from mid 120's to mid-high one-teens. I maintained the tiny frame until 1992. (I'm thinking this is where I may have done myself some damage...) After 30 and with two additional C-sections just 20 months apart, plus burning the candle at both ends, my youthful vigor started to slip away. I still maintained at roughly 140 for the next fifteen years, though.

To cut to the chase... in the last TWO YEARS I have put on roughly 30-40 lbs... by medicating with sugar. I have taken twenty of them off, but my lower abdominals are just SHOT. The girl who used to do a hundred and fifty situps a day has become the woman who finds it near impossible to do as much as ONE crunch. I can do a lot of OTHER exercises, but lifting my body from the floor into a crunch just isn't happening!

I desperately need something to strengthen my lower abdomen... the interior deep muscles that were cut and cut and cut and cut and stretched and stretched...

Help?

Try researching Callanetics.

Planks. http://www.youtube.com/watch?v=kiA9j-dR0oM

Planks are good.

Pilates core work has a lot of variety. I have a DVD called Body Target Abs. It has two 20 routines. One is yoga and the other is pilates. I never do the yoga one although it's effective as well.