As a runner, I could say the same about running a marathon. A half-marathon (or near) distance was practically a light exercise for me for a long time there. A marathon would be a harder exercise, fatiguing, but not too challenging. Because I was prepared (running 5/10/15k regularly for several years). But I would never call it "just running" for anyone else.

The idea that hiking is "just walking" because for you it was, is similarly absurd. With elevation changes, gear on your back (10-40lbs depending on how lightweight you go, easier to be lighter than when I started with metal-framed packs), significant distances per day, potential time constraints requiring longer daily distances, you have similar "heavy stressors" in hiking.