Does your body adapt to running in the sense of calories burned?

Like say you're totally out of shape, and you go run 1 mile in 12 minutes. You're all tired and out of breath when you finish, and you're pulse is racing. You have just burned x calories.

Say you keep running for months and months. You get better, etc.

Now, you go run 1 mile in that same 12 minutes. Am I correct in assuming that you've just burned less calories than the first time you did it? Your heart rate is lower now when you finish, so that means less calories burned, right?

So if you want to burn a lot of calories when you run, you have to keep increasing the demand placed on your body, right?

My friend wants to know, and I don't know much about running.

Thanks,

IronFist