Stairs are different muscles. I used to work at a dam where I would have to climb 40 flights of stairs/ladders multiple times a day with 80 pounds worth of tools on me. Before then stairs were difficult for me, now I can run up that with that much weight no problem. I haven’t worked there for a year but I also can do sub hour 10k (barely) but those muscles stay with you as long as you stay on your feet regularly during the day.

If it’s an issue for you I suggest weight training up and down the stairs you have available to you (in your house maybe?) 5 minutes a day with a couple 20 pound weights up and down those bitches and you’ll make walking stairs your bitch for the rest of your life. If you can do a sub 10k you have the willpower to do it if you want to.