I’m a runner. So why do I get winded by one flight of stairs???

It used to happen to me all the time at work. Running late for a meeting and it’s only one flight up, so I dash up the stairs. I then arrive at the meeting huffing and puffing like I’d just run a mile. It was ONE LITTLE FLIGHT OF STAIRS! In my lateĀ 20s, I was … More I’m a runner. So why do I get winded by one flight of stairs???