So glad you loved MP! Ngaire is right - get out of Cusco early if the altitude bothers you.
Editing to add fog delays on flights to & from Cusco are fairly common. Book the earliest flights out to minimize delays.
Also heard yesterday Peru is suing Yale to recover many of the artifacts it has from Hiram Bingham & colleagues trips to Peru in the early 20th century. If you are tempted to see MP, go!
Cheers, Anne