[carmad]

My experience in Peru on a medical mission as a relatively fit 40 yr old was interesting. Was in Arequipa (7600 ft), about the same as Machu Picchu. Resting heart rate 82 bpm, oxygen saturation was in mid 80's (normal in high 90's).
Altitude sickness is no joke, used Diamox to help symptoms and sucked on Coca lozenges, which helped somewhat. You will fatigue easily, stairs will make you breathless and activity more challenging, however everyone's body will react differently. Cusco is at much higher elevation (11,000 ft), actually got to about 15,000 ft during trip to Colca Canyon which just sitting drinking a coffee I was breathing harder. Stay hydrated, don't push yourself to hard, and enjoy a beautiful place.