If you honestly think about what you are trying to accomplish, you will see the difference between what you want and what you are doing.
Sit on your ass all day? Stop sitting on your ass all day! Run for 30 minutes EVERY DAY. I don't care if you have to stop and breathe often. Start by walking 5 minutes or so for now, or jogging. Then start running. I don't care if you walk it or jog it or run it. Just do it for 30 minutes a day. Try to do it every day, that way it well end up happening about 4-5 times a week.

And you know what, it's really not hard. 30 minutes that you will love and feel awesome after.
But remember, try to push yourself. You don't want a heart attack, but don't make it easy. You're not supposed to feel EXTREME PAIN, you're just supposed to raise your heart level. This will get easy fast, however. Each time you run, you will have to run a little harder or a little softer based on how you feel. Make the most of those 30 minutes!
After about a month you will see much better running form developing. A human body adapts to whatever tasks it is doing. So if you are run frequently, your body will adapt and you will start looking more and more like a runner. Simple biomechanics. Your posture will improve. And future runs will get easier and easier. Not only that, but you will be losing fat.
And if you want to
build muscle, DON'T! Simply MAKE YOURSELF RUN on a treadmill or outside until you lose weight. Your problem isn't muscle. Your problem is diet and a sedentary style of life. Lose weight and love it, and then build muscle afterwards when you're skinny. The muscle you build afterwards will help you have an easier time staying fit since muscle
burns calories. But the simple answer to 'sitting on one's ass all day' is to change that!
As for the diet, stop eating ****. Drink many glasses of water and gatorade. Drink 3 cups of water before your run. Also drink water during and after your runs. Don't eat after 6.
The other thing I want to say is that even without exercise, a body has no choice but to use up fat for energy if you start eating healthy, since it won't be getting calories via the mouth. But with exercise, it will be even easier. You understand math, it's this simple. There's no magic or genetics or anything to worry about. Here's a program for you:
bool amBurningFat (int int****YouEat, int intHealthyFoodsYouEat, intCaloriesYourBodyBurns) {
// total up the different calories from junk food + good food
int intCaloriesYouConsume = int****YouEat + intHealthyFoodsYouEat;
// does our body use up more calories than we eat?
if (intCaloriesYourBodyBurns > intCaloriesYouConsume)
return true; // YES! LOSING WEIGHT!
else
return false; // same exact weight or gaining weight!
}