I have not done Greece or Sicily, but we did just get back from a Collette tour to Northern Italy. The Collette website shows the activity level, rated 1-4 for every tour. We did to a MVC exclusive tour, as that had the cities we wanted, and was only a level 2. My wife has some mobility issues, and wanted a lower level of walking. I would say the tour worked well for us. Europe in general does have more stairs than US, and at least one gentleman on trip did have some challenges with the stairs. I would suggest you read up on the different descriptions of the different activity levels, and find a tour that works for you.
I can say that the tour was great, and Collette was great to work with.