Calories and Exercise

The way MFP is set up is that you are supposed to eat back some or all of your exercise calories. If you daily goal is set at 1200cal and you did 900 cals of exercise you can eat 2100cals and still be in an overall calorie deficit (as your net calories are 1200.) some people find MFP over estimates exercise calories so they eat back about half the calories earned by exercise. So for yourself that would give you 1200 + 450 cals per day = 1650 per day.

Your calories are way too low for your fitness level.

Here's what I advocate if you are a regular exercise person (as you are, and so am I)

Instead of restricting your calories to some very low level, eat at only a SLIGHT caloric deficit from your maintenance, and create the remainder of your deficit with exercise. You're still going to create a good deficit, due to the exercise, but you'll have fuel to perform as well.

My maintenance, for instance, is about 2,000. Now, I could drop my calories to 1500 per day, for a -500 deficit just from diet alone, but that just isn't enough calories if I am exercising.

On gym days, I will burn, like you, about 900 calories total (gym + my 4-mile daily walk). On 1500 calories that's just not feasible or maintainable. However, if I eat, say 1900 calories instead, and still get the 900 from exercise, I'm at a 1000 calorie deficit for the day. That's double the deficit I'd create if I didn't move all day, sat at my desk, and ate a lousy 1500 calories.

Exercise is great, for all the reasons I'm sure you already know, beyond just creating a deficit. So FUEL YOURSELF! Give yourself the calories to exercise. Eat at a slight deficit, and make the rest through exercise. You'll be glad you did. The energy you'll have will be well worth it, and you'll still lose weight.