I had a tummy that looked like collapsed dough for about 2 years, I walked the dog for about an hour a day which kept me reasonably fit but it still took that long for the tummy to start looking less saggy. You had twins, which must have a more profound effect!
I started running regularly (at least 3 times a week) at the beginning of last year, built up slowly and became very fit. The tummy looked fab. Unfortunately I am now nearly 38 weeks pregnant!
As you have no injuries, I'd consider doing pilates for core strengthening and either brisk walking or gentle jogging to burn calories as often as you can - or join a gym class (I hate them but they should be able to give good advice).