Dr. Hibberd's answer:
 

The protuberant belly many of us develop as we age is associated with deconditioning and poor exercise choices over an extended period of time. While cirrhosis may cause fluid accumulation in the abdomen and enlarged livers are often evident in heavy drinkers or those with right heart failure, their enlarged bellies reflect serious underlying diseases, not just abdominal wall muscle laxity and deconditioning.
 

See your personal doctor to be sure you don't have an underlying condition. If you are otherwise healthy, I suggest you seek the advice of an exercise professional to be sure the exercises you are doing are targeting the correct muscle groups to return you to a more youthful, improved abdominal wall muscle tone and condition. You are going to have to be patient, because the time this takes will be measured in months not days.