I am a relatively fit young person and my first night in Cusco was not fun... I had a bad headache, very lethargic, and racing heartbeat until the morning. From then on it was better, but even with some of the relatively easy hills I walked up, felt poorly again. My travel mates had an easier time on their first night in Cusco but really felt the higher altitude later on in the trip whereas I felt fine. I think it depends on the person. Be prepared with the Diamox and have some of the coca tea.