Which specific cruise or cruises are you doing? In general if the included tour states it is "panoramic" (for example Panoramic Athens & Archeological Museum), then generally that is a bus tour driving by the sites with maybe a photo stop or two. I will state in that example, we just returned from a 10-day trip to Greece (non-Viking) and spent 5 hours at the Archeological Museum on our own - however, there are a few seats around the museum to rest a minute as well as a cafe with chairs/tables. We observed a couple of ship excursions touring the museum while we were there and of course their guide just takes them to the highlights, while we were looking at everything! 😉 Viking has this included tour labeled as Moderate.
Since you mentioned Turkey, I expect you are likely going to Ephesus. I just checked the Viking Included tour and it is called "A Gentle Journey into the Ancient World" and keeps you on the lower part of the archeological site which is much easier walking than the upper part. We were just there Oct. 1 and I don't recall many places to take a rest. Viking has this tour labeled as Easy.
If it is a walking tour, such as at Rhodes (Rhodes Town Walk & Palace of the Grand Masters), it too is listed as Moderate activity. So, my advice is to carefully read the descriptions of the included tours and check the activity level mentioned. Let us know which specific cruise you are on and people may be able to give more specific info about the included tours.
If you do decide to go, you might want to get a set of walking sticks to use on excursions instead of the cane. I recently purchased a pair on Amazon that are quite lightweight, sturdy, and collapse small enough to fit even in a carry-on for only about $30.
Looks like a great trip and I hope you are able to go and experience it to the fullest!