Why not try the same kind of discipline with them that you impose on yourself in regards to how much time you spend in the gym? You expect it of yourself, so why not expect it of them?
Ah yes - they don't respond to the same kind of self-discipline you do? That's right. Neither do many of the rest of us, actually - not everyone wants to be a hot mama just like not every child will respond to the same form of discipline.
What you do is to take yourself out of the equation and learn that what you want doesn't necessarily translate to others, and be accepting of it. You work with your kids in a way that helps them understand your expectations, you give them consequences that apply to the situation, and then you help them see that THEY are in control of themselves. The 9 year old leaves her clothes all over the floor? Pick them up and put them away until she doesn't have enough clothes to wear. She complains? You tell her that she didn't put them in the hamper like she was told to do, so she doesn't get them back. It is REALLY simple and SHE is in control.
The 6 year old hits the 4 year old? Send her to her room. No computers, no game boys, no XBox, no electronic games of any kind in the bedroom. If the kids fight, they are all separated to their rooms without you deciding WHO started it. They will get sick and tired of being sent to their rooms and will eventually get along better, IF you are 100% consistent. IN fact, they will police each other so that they don't ruin it for the rest of them (like the Army...)
Part of being a good mother is to handle your children with more conviction than the time you spend on yourself. If you do a good job with them, you'll have more time to spend on yourself in the long run. And you'll have better behaved kids.
Dawn