Those stairs have made you stronger over the years. I will keep going to my second-story home office until I can no longer move my knees. Keep climbing them!
From AI:
Stair climbing builds strength by forcing the body to lift its own weight against gravity, activating major lower-body muscles like the glutes, quads, hamstrings, and calves. It acts as a form of high-intensity interval training, improving cardiovascular health, increasing bone density, and enhancing balance, which can reduce injury risks.