The only way to flatten ones tummy is to lose fat.
To lose a kilogram of fat your body needs to burn ~7700 calories. Running 5km would at best burn a few hundred calories depending on your fitness level. If you want to lose weight exercise is not the way to go, a marathon runner can be a fat ass, but this does not mean that working out is shitty idea. If you want to lose weight the only thing you can do is eat less calories, than you consume via your basal metabolic rate + the amount you expend working out.
Also, imagine fat in ones body as a lake, losing fat is in a sense like taking a bucket and taking the water out of it like that. Meaning, you lose fat from everywhere, not from where you want. Now there are a lot of holes in the lake, meaning that those regions have more water in them. It's the same for your body, the location "of the wholes" is based on your genetics. So don't expect to become lean everywhere you want to anytime fast. You cannot concentrate weight loss on your stomach, the only thing that can do that is liposuction.
Ab machines are a waste of money. Go buy a pair of dumbbells and start eating well for the same cost, it'll be far more useful.