Everything is in the elevation.
I'm rather fit, used to be in the army nearly ten years ago. These days I hike now and then. Last weekend I went on a wee tip, 25 km and 1400 m elevation; it took me 9 hours, not including my lunch break. Only carrying water for the day, the said lunch, dry clothes and a bit of gear ; probably a 10 kg pack. Mild weather and decent track. On flat ground, I would have halved that time.
