So you can take long walks, or you can forget about the elevator from time to time and even walk to your office instead of driving. It will be better than just sitting on a chair all the time or sleeping all the time.
I personally worked in a place where I had to stay sited on a chair for more than 8 hours. We always joked that one day we will own asses of the size of the chairs. The main problem is that we were right. Quite fast this phenomenon happened. Even so, to develop a lean muscle mass you should make a little bit more effort than that.
So why those thin people never get fat?
The answer is genetics. They really own a good inheritance from this point of view. If you see a person who weighs less than 50 kg and has not visible fat deposits either and still eats all kinds of junk foods whenever they feel like it, no matter the time of the day, they must have a great genetic inheritance regarding the metabolism’ speed. They should not be accused of sorcery.
If you have weight issues and you want to finally own the body of your dreams you should stop doing nothing about it. You should not find excuses that you were born this way. Your metabolism is working perfectly if you do things in the right way.