It pretty much has to be. 1kg would be 1L of water, which is not that much to lose over 5 hours.

But 1kg of fat is 7700 kcal, which would be a massive amount of energy to expend in 5 hours. Unless your "walks" involve running uphill with a 25kg pack or something like that. :)

Short-term energy consumption on a 5 hour hike would be from stored glucose, fat breakdown IMHO would happen slower. A 20km hike can consume 1000-2000kcal depending on how hard it is (climb, terrain, weight of hiker, weight of pack matter) so that would be 0.25-0.5kg of carbs - so potentially that loss could be 50/50 carbs and water.

But hydration definitely is a big aspect of mass simply because of how much you add and lose. I once did a ~90km hike in 24 hours or so, and I believe I drank something like 15+ litres of liquids during that.

Depends on what you’ve eaten recently. It’s pretty easy to burn fat on a hike if you go while fasting.

Humans lose moisture for a variety of reasons normally, you don’t need to be sweating in order to lose it, although that obviously drives the loss rate up. You lose a ton of water through just breathing.

On your walks you’re probably burning glycogen, the carbohydrate we store in our liver and muscles for easy access energy, and exhaling a lot of water. We store about half a kilo of glycogen which is then bound up with 3-4x as much water still. While you’re not likely to consume all your glycogen in a 20km walk (you need a marathon to do it), burning a decent chunk of it on top of normal water loss isn’t an unreasonable hypothesis.

Yes. A lot of fad diets and supplements work on this principle; dehydrating you appears to lose weight really quickly initially. It's not sustainable.

The human body is darn efficient.

You would probably have to run 50-70km to lose 1kg fat.

It's easier to not eat calories than to burn them after eating.