However, some new runners find that they don't lose weight, some even gain weight, or they lose a few pounds and then hit a weight loss wall. There's no simple answer here because there could be a few things going on.
Too Many Calories
First, it could be you're eating more calories than you need. Even if you're running, if you aren't burning more calories than you're consuming, you're not going to see a difference on the scale. You may be hungrier than you were before you started running, and you're eating more calories than you realize. Try spreading out your calories throughout the day into 5 or 6 small meals so you don't get ravenous and overeat. And make sure you're snacking on healthy foods, not junk food, which can trigger hunger.
You should also watch your liquid calories. Although you may running a lot, you don't need to constantly drink sugary sports drinks. While it's important that you replace electrolytes during long runs, it's not necessary during shorter runs or when you're not running. Plain water is fine for staying hydrated. Try to also limit consumption of fruit juices, specialty coffees, and regular soda since they also add a lot of calories to your diet, but don't make you feel full.
You should also make sure you know exactly how many calories you need each day, since the USDA's 2,000 calorie diet is only a recommendation. Use this daily caloric expenditure calculator to determine how many calories you actually need each day.
Muscle is Denser than Fat
Another explanation is that you're building more muscle mass, which is denser than fat. So while you may not be losing weight, your body fat percentage has decreased and you're more toned than you were before. Try to pay attention to how you're feeling overall, and use measurements other than weight -- such as inches lost or how your clothes fit -- to mark your progress.
Determine Your Calorie Needs
Knowing how many calories you need will help you figure out how much you need to create a deficit of 3,500 calories, which is equal to one pound. So, if you're running 14 miles a week, that means you're burning about 1400 calories a week by exercising (assuming you're not doing other exercise). To get to a 3,500 calorie deficit, you would need to cut 2,100 calories a week, or 300 calories per day, to lose a pound a week. If you've been getting your recommended amount of calories, or even going over, that would explain why you're not seeing weight loss.
Add Distance and Intensity
You'll probably have more success if you increase your overall weekly mileage. If you always run the same pace, try incorporating speed intervals in one run. You can start adding speed by warming up for a mile and then running at a faster pace (breathing heavy but still in control) for a minute, and then recovering at an easy pace for a minute. Continue with this pattern for two miles, then cool down for 5-10 minutes. When that gets too easy, you could always increase the time of your speed intervals or do hill repeats instead.