It's easy to get overwhelmed by all the numbers.
My advice to you would be not to overthink things, first of all.
My next bit of advice would be not trust the calorie calculators - even though they take into account your weight it's really really hard to know how many calories you're burning w/out measuring your heart rate and knowing what kind of exertion you're putting out. 800+ calories is an awful lot at a walking pace, even for 90 minutes of exercise. I wear a heart rate monitor that takes into account my heart rate, age, and weight, and when I *run* hard for 30 mins (at 5.6 miles an hour and 1.5 incline), I only burn around 400 cals. That's not to say that you aren't burning that many calories, but I'm skeptical. It's hard to say.
If you are, then you definitely need to eat more to support that level of exertion.
Here's the thing: When you reduce calories, you are going to lose fat, but you're also going to lose some lean muscle. It's pretty much unavoidable - it's just the way the body works. You can avoid losing a lot of lean muscle by exercising (including resistance and weight training) and eating not just a healthy amount of food, but the right kind of food - lean protein, veggies that are rich in nutrients, and complex carbs and whole grains.
When you eat too little, or you don't eat the right kind of food, you lose MORE lean muscle, and you lower your metabolism, and then you get into a vicious cycle
Ok, now, on top of all of that is the lovely fact that even though it's a mathematical equation: [calories in] - [calories out], our bodies are complex enough that it isn't that linear.
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There are all kinds of other things that affect how fast you lose weight - everything from how your body handles carbs, to whether or not you ate a lot of sodium and are retaining water, to your time of the month, to the barometric pressure outside, to the state of your metabolism. (I know .. it sucks - would be so much easier if you could just manage the numbers and let everything fall into place!)
So here's my advice for you: First thing is let the numbers be a guideline, rather than a goal. Figure out what a healthy amount of calories is for you, and drop that by 30% to get your "diet" amount. The calculation I use (and the average for someone who is moderately active) is around 15 cals per pound of bodyweight. So for example, I weigh 174. If I multiply that by 15, I get 2610. Drop 30% off of that and I get 1827. So my "weight loss" calories are 1800, give or take a few.
Then I exercise. I don't worry about how many calories I burn while exercising or try to calculate them or anything like that. I just do it. I lift weights 3x a week and I run 3x a week and I take a day off for rest and recovery. You don't have to have the same routine, although my suggestion would be to add in some weight training or resistance training with things like squats and lunges and so forth, to alternate with your treadmill work.
Start out like that and give it a good 6 weeks to see what happens. If you're not losing a healthy amount of weight, then drop your calories by 200 and give it another 4 to 6 weeks.
Remember too, that a healthy amount of weight loss is about 1% of your current bodyweight per week. Again, for me, that would mean to be healthy I should be losing about 1.7 lbs per week, give or take some.
And remember that we're all different, so you're going to have to pick a starting spot and tweak it here and there. And as you lose weight and your body becomes accustomed to working out, you're going to have to tweak again. It's a moving target ... not a linear path.
I hope that helps you a little and hasn't confused you more. Feel free to ask questions if you need to. I"m happy to answer.