The No Pain No Gain is very attractive. I'm sure that the only ones who have found this to fail are those who haven't done it enough, or not done it properly, or any number of other faults with them.
If your stomach is sagging it's because the muscles are not sufficiently toned.
To tone up your muscles, practice, pulling them in. Quite hard, for perhaps 30 seconds at a time. Then rest, for at least 10 to 15 minutes, until you feel completely rested.
Never hurt yourself Never over do it. Avoid when walking, it's best either sitting or lying down. And never become obscessed. It will imporve in time.
As for weight, the evidence connecting weight to food intake is not particularly convincing and full of anomalies that the diet industry doesn't like mentioned.
But, Never miss breakfast. Never miss an evening meal. If you are heavy, eat smaller portions. Eat slowly.
Get yourself into a mind set, by convincing yourself you will be better having a little less. But remember you are who you are. If you're not a film star then be glad of that.
Stop worrying about it. It's mostly a money making con anyway.
OK?