In reality the only real way to tell how fit you are is to see how much exercise you can do. You will find that if you begin an exercise program and begin to increase your fitness significantly, your weight will come down.
Diet, although it is important, is often not the most important path to losing weight. If your goal is to slim down and avoid putting on weight again, you should examine how much exercise you get. Obviously you should look at your diet also, but often the most effective treatment is to work off the food you are eating. Making time for exercise in your day and making sensible choices on food are not two different ways to lose weight - they should be done together.
When you start exercising you dont just burn fat, you also strengthen and tone your muscles. This will increase the rate at which you burn fat also. It can be disastrous to start out dieting without a corresponding increase in your exercise. This will often end up burning your muscles instead of your fat. So you will lose weight, but not fat, and in fact your body will naturally burn less fat as a result of having less muscle. This will make it harder for you to exercise and lose weight in the future.
If you get exercise on a regular basis the fact is you will be fitter than someone who gets no regular exercise, regardless of your weight. Taking this into account you should start exercising more when starting any new diet plan, this will get the most from your new diet.
If your goal is to improve your fitness you will need to establish a proper exercise regimen for yourself - a little exercise now and again isn't enough. You'll need to combine cardiovascular or fitness exercises with some basic strength training.
Cardiovasular exercise is exercise that increases your heart rate. When doing this form of exercise your goal should be to increase the amount of time you can exercise for. This will increase your stamina and fitness.
Strength training will tone your body and increase your strength. While it won't necessarily make you fitter, it will make you look and feel better and give you visible, confidence-building results.
This does not mean that you have to become a weightlifter, or that you need to work on building bulging muscles. In fact, just a little bit of strength training should be enough. You don't even have to invest in weights to lift - there are plenty of strength training exercises that you can do in your own home with no equipment.
In the end the most important factor in your fitness is whether or not you can do as much exercise as you'd like to be able to do. If you can then you are fit, if not then regardless of your weight the road to fitness need not be difficult, all it takes is a little determination.