Cusco is 3,600m above sea level so coming from Lima I had to adjust to the altitude. That was my first three days right there. Although I got off lightly compared to other people I spoke to, a drastic change in altitude took it out of me and I spent the first day doing absolutely nothing and the next two days doing not much.
I heard that breathing at high altitude burns the same amount of calories as jogging at sea level. This explains the exhaustion that comes with adjusting to altitude. I even got breathless going for short walks which was frustrating for me as my favourite way to get to know a city is just to wander for hours!
I did have an incredible day exploring the Sacred Valley though, on a tour which included the towns of Pisac, Ollantaytambo and Chincero. The drive was stunning as we wound our way through the valley.