The altitude at Machu Picchu wasn't really a problem. I didn't find climbing the (many) steps there much harder than climbing the volcano on Bartolome in the Galapagos (about 114 meters). Cusco does feel very different. I find myself drinking even more water than at Machu Picchu, and find myself occasionally holding my breath (blowing off too much CO2?) especially when I'm falling asleep or resting. It's also very dry here and my lips and nose are getting chapped. It's OK, I just have to concentrate on taking it slowly and keeping hydrated. Don't know if the coca tea is helping but I actually kind of like the flavor.