Your body temperature goes up as you walk. Try it in winter - when you walk you stop being cold and may even start sweating.

It doesn't matter. "Calories" don't turn into heat. All turn into co2. Heat is just a side effect of burning.

You do not exhale pure CO2. The CO2 in exhaled air can rise a lot before you need to breathe faster.

No, it can't. Percentage of CO2 in exhaled air is pretty much constant because it's a result of the physical process body uses to get rid of it which can't really be scaled up to higher concentrations of co2 as there's really no reason to scale it down from peek efficiency.

Your lungs can take "deeper" - i.e. more voluminous breathes. A person sleeping lightly can breathe quite shallowly just fine, a person up and walking about (in decent shape) will breathe more deeply - professional athletes for example don't breathe as quickly as an unfit person.

Which is to say, it's too simplistic a model to talk about "breathing twice as fast" as explaining what's going on.

Ah, yes, volume matters. I probably should have said "breath twice as much" not "twice as fast" but the rest still stands. And you probably breathe twice as much when you are walking than when you are at rest because moderate excercise (stationary bike) makes you breathe about thrice as much.

I'm curious about nighttime vs daytime breathing. I was under the impression that sleeping person breaths deeper than resting person. At least you can tell the exact moment somebody falls asleep because they start to breath deeply and loudly.