What a flight of stairs actually is
Take it apart mechanically. Each step is a single-leg movement: one leg lifts your entire body weight against gravity, then the other, repeated a dozen or more times per flight. That is load-bearing resistance work by any honest definition, the same family of effort as the weighted step-ups people pay to do in gyms, except the weight is you and the gym is the building.
Your heart rate rises by the second flight. Your legs notice by the third. This is not a metaphor for exercise. It is exercise, disguised as architecture. Nobody needs to be taught how to do it, and nobody needs to warm up for one flight.
Why people avoid them
Ask someone why they took the lift and the answer is rarely physical. It is social. The lift is the default, and the stairwell feels like a service area, a place for fire drills and people who have been locked out. Arriving slightly out of breath feels like confessing something.
None of these reasons survives inspection, but they do not need to. Defaults never argue their case. They just stand there, humming, with the doors open. Nobody ever chose the lift. Choosing begins when you walk past it.
Making the climb automatic
The way in is embarrassingly small: one flight, then two. Climb the first flight of whatever building you are in, take the lift for the rest if you need to, and stop negotiating about it. Two flights takes perhaps forty seconds, less time than the average wait for the lift you were standing at. The negotiation is the enemy. A habit that has to be re-decided every day is a habit that will be declined on wet Tuesdays.
Within a couple of weeks the stairs stop being a decision and become simply the route. The habit is the whole intervention. The fitness is a by-product that shows up later, uninvited.
Start with one
One flight a day, every day, for a week, before you add anything. This will feel too small to matter, which is exactly why it works. There is no version of your life, however busy or tired, in which a single flight of stairs is a burden.
After a week, make it two. After that, the building starts suggesting numbers on its own. People fail at exercise by starting at the size they think they should want. Start at the size you cannot fail, and let the stairs do what stairs do.
The stairs were training all along. You just had to stop taking the lift long enough to notice. One flight, tomorrow, on purpose. The lift will still be there if you change your mind.