Yes the altitude is a concern. As I recall, the HAL literature stressed this point, warning people with a possible problem, not to take the trip.
We visited the crater on Maui (10,000 feet) a couple of years ago, and did not have a problem. (My wife and I are 66.)
The city of Cuzco is at 11,000 feet. Our tour included a visit to the Inca Ruins (Sacsayhuaman) above Cuzco, which are more like 12,000 feet. However, Machu Picchu itself is at 8,000 to 8500 feet.
The tour around Cuzco involved walking on mostly level areas. However, at Machu Picchu there is a lot of difficult walking and climbing on irregular stone steps. Therefore, if you have an altitude problem, it will probably be at the ruins, not in Cuzco.
(The natives told us to chewing coca leaves will help with the altitude. When we checked into our hotel, we were immediately served coca tea, with the waiter telling the tea will help. It was apparent to me that us gringos didn’t care for the tea – it must be an acquired taste.)