By all means check with the office for details on specific tours.
Your concern is not the distance you can walk but rather the pace of the group. 4-5 miles in a day would be a very busy day for most tours. In general, I have not found the tour groups to move very fast. The logistics of keeping 25-28 people more or less together necessitates a leisurely pace and frequent stops and, of course, there are usually frequent stops to talk about the sights. The guides are well aware of distances and difficulties of specific tours and one always has the option of sitting out any particular activity. Nevertheless, all Rick Steves tours will have a lot of walking every day, often on hills and uneven ground.
Note also the requirement to be able to handle your bags and walk a fair distance from the bus to the hotel. Can you do that with a cane? Can you use a backpack? I use a roller bag and sometimes find myself cursing the cobblestones.
Most hotels have elevators, often very small but usable. The office should have this info. Also, the guides assign rooms so you can ask to get a room on the lowest floor.
Hope you are able to do it,
Robbie