Wow. First welcome aboard & congrats on your 80 lb loss!! That is no small accomplishment! The plateau has probably resulted either from your body having adapted to the diet & training you have been doing since the beginning of the weight loss effort. Or it is your body need to take a break to get used to its current situation where you have lost a lot of weight in a very short periold of time.
Are you currently working with a nutritionist or a training? Can you post your current diet (take a typical day's menus & times of eating). And do you do any resistance training? You are doing huge amounts of cardio - describe the type of cardio you are doing. Is it the same thing on all 5 days?
If you haven't done any resistance training, I would highly recommend that you start. I'll hold my request to rush over & buy a gym membership until I hear your situation, but doing only cardio will help you lose fat, but you will also lose muscle if you only do lots of cardio. A big part of getting a really great muscle / toned look is by building up your muscles as well as by cutting bodyfat. Note that your bodyfat measurement is a ration of your body muscle & body fat. You can drop the ratio by dropping body fat, but you can also drop it by increasing muscle. To illustrate, consider what your typical long distance runner looks like - very lean & almost skinny, sort of stringy mucles. Then look at what a sprinter looks like. Lean & muscles! Who looks "healthier"? (No offense to the long distance people .... just trying to make a point.) If you continue doing nothing but cardio, eventually you will approach the long distance runner look. And also if you get stuck at a certain bodyfat measurement & adding more cardio won't get you anywhere, you can throw in some good weight training & create a more toned look by buildign up the muscle.
Let me give you an example of that - my mom is 63, she walks 5 miles/day & step aerobics w/ a little bit of resistance training (3 lb weights) in the class 3x/week. She eats a couple small meals / day, usually more fruit, light on the protein & loves pasta. She's 40% bodyfat. She knows that number & it freaks her out given the lifestyle she follows. But I can't get it across to her that if she eats a little more protein, I think also her fats are not sufficient, she again freaks out - she thinks too much protein will get her to freaky & I still think she considers "fat" in the mindframe that started the "low fat" fad. Also she doesn't want to "lift weights" because she thinks women don't do that & they can't lift heavy weights. When I did a couple local body building shows a couple yrs ago, she was convinced I was going to look like Arnold Schwarzeneger. So the misconception is already established.
This drives me fricken nuts.
So back to you - give us your typical day's diet & training. Personally I would say it is time to make some changes in your routine and also take a close look at your diet because 800 cal/day is starvation. Your body may be thinking that it is in a "drought" situation and is going to hold on to all its resources (fat) like its the last meal on the planet. The idea that you have to starve yourself to lose weight is very wrong. You have to eat to lose fat - but you have to eat in an optimal way, such that you feed your body what it needs at intervals during which it will metabolize what you fed it and then provide it more when it is ready to start metabolizing again. This is the basis for the idea of eating 5-6 times per day. If you feed it too much, the excess will be deposited as fat. If you feed it too little, the metabolism function will slow down (e.g. like when you see your rate of fat loss slow down or stop) and your body may also turn to your muscle as an energy source.
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