There really isn't an accurate number that someone can give you because it depends on how fit you are and your genetics. Your level of fitness will determine your level of effort and the more you do endurance training(like running) the more your body adapts. One way is a lower heart rate when you're exercising at different intensities. Additional factors that make a difference are your amount of muscle mass and nutritional habits.
Get a HRM. I love mine!
Running, believe it or not, uses the body's energy in a different way. The leg muscles has tons of stored energy, your body burns that first, and then it starts using fat as energy. If you run too much, your body can bypass fat and use muscle instead as energy. This is why you shouldn't run for a very long time.
6 miles run? that is around 500-600 calories
6 miles run with a metabolic enhancer (green tea, coffee, orange juice) is about 1000 calories
depends on heart rate and fitness level. but 500-1000 is a good range.
Check it out, and it'll let you know. But I don't think it'll factor in gradiants. So it'll assume flat ground.
Its hard for me to guess because I'm waaaay bigger then you. I'd be 800-1000. maybe you're 500-600?