If you want to get a standard estimate of the calorie needs for the average person with your stats, use the Harris-Benedict calculator at
http://www.jimkaras.com/loss_math01.cfm. Of course, your needs may be different, but this is a place to start. Eating 1400-1500 calories may be OK for you, but that may be too few and that will cause your metabolism to SLOW DOWN. It's just as important to eat enough as it is to cut enough to allow weight loss.
Also, make sure you are eating breakfast and that you're eating at least a small snack every 3 to 4 hours. The less frequently you eat, the slower your metabolism. Another thing you can do to boost metabolism is to take a couple of 10-minute brisk walk breaks during your day. The 30 minute aerobic walks you are doing are GREAT, but by adding a little activity throughout the day you can keep your motors running, and that will burn more calories in the long run.
Sooner or later you will also want to add strength training to your routine, to build lean muscle. This not only "tones" your body but -- you guessed it -- boosts your metabolism. If you are easing into these new habits don't stress about it, but eventually you'll want to start to get the best results possible.
Good luck, and congratulations on making this effort. The results are worth it, even if you do hit these patches of frustration sometimes.