It's definitely the thin air you notice most. The climb itself was pretty mild, a little rock scramble approaching Class 2, here and there, but nothing terribly challenging. But your heart is pounding in your ears pretty much the whole time. What is amazing/infuriating/awesome/disgusting is to see the "I'm training for the Pikes Peak Marathon" crowd. These studs (and studettes!) literally scamper up this with nothing but a few ounces of water. Full-on beast mode!