altitude sickness in Ecuador

My husband and I are considering the March 6th trip.  But, I am concerned about the altitude sickness. When we went to Peru, I had to wear oxygen mask when we arrived at Cuzco (the sickness lasted one night, luckily).  I was wondering if altitude sickness is common among tourists? How safe is the city Cuenca? We both are seniors and do not wish to be in a city that is not safe.

Can someone provide experience living there?