Here's a chart:
http://www.disabled-world.com/artman...ht-teens.shtml

You're about 5 feet 4 inches tall, and 140lbs. Chart says average is 115 – 120 lbs. So I don't want to sound mean, but maybe you might want to think about doing some more intense cardio work. How about running? When it comes to losing fat the idea isn't really to push yourself to the point where you feel like your about to pass out. I'm actually under average weight on that chart...lol. 5'9" 140lbs.

Take 1hr 3 times a week. Basically start with a light 15 minute warmup jog just to slowly get your heart rate up. From there do 30 minute of running in your target heart range. After that take a light 15 minute jog to cool yourself down.

Your target heart range is the point where you'll burn the most amount of fat. Any higher and you're just pushing yourself too hard and you're not going to burn anymore fat. Lower than this and you're not working yourself hard enough. Invest in a decent heart rate monitor. There's plenty of decent priced heart monitors. What you want to look for with a heart rate monitor is that it has a chest strap. They will be more accurate than the watch taking it directly from your wrist. Polar is a pretty good brand, look up reviews and determine what you're willing to spend.

Here's a calculator to find your target heart range.
http://exercise.about.com/cs/fitnesstools/l/bl_THR.htm

Also if you don't feel like paying for a watch you can do the talk test. Basically if you can hardly talk, you're working too hard. But if you're breathing has not increased and you're not sweating, you're not working hard enough. I've gotten to a point where I know where my target heart range is pretty accurately. It's fairly easy to tell once you've done it a few times.

Hope this helped.