I feel with your description of yourself, you would not have any problems with this tour. My husband and I just took this tour in April of this year. We are 58 and 69, we too walk same amount, but daily. We have two dogs and they help keep us in shape! I did quick math calculations and we averaged 5.87 miles per day, but this included walking we did on our own during free time. That said, I did not feel the tour was overly strenuous. Of course there are hills to climb, the Greeks built their temples perched atop said hills. Each town we visited was hilly. I reviewed the itinerary and noted there was only one day marked as strenuous with the rest mark as moderate. From what I remember,about the strenuous day, there was a lot of walking, morning walk through market, walk in cathedral and cloister, and then walking in Palermo. There were several towers with stairs to climb. Also, the day you walk in the Valley of the Temples is probably the longest walk, and over rough terrain in places. But, we stopped quite a few times to listen to the local guide. I got sunburned that day, but was not overly tired.
You did not say which tours you took in Italy. If you have been on Village Italy or Southern Italy, I would compare Sicily Tour to those when it comes to activity level...hills, walking in museums, and around Greek or Roman ruins.
I hope this helps and that I did not ramble on and on without providing any insight.