Fitness level for guided tours: how much walking is there really?

My parents (both 70) want to do a guided tour of Italy but my mom has bad knees and can only walk for about 30 minutes before needing to rest. The tour says “moderate fitness level required” but what does that actually mean?

How much walking is typical on a guided tour per day? Are there usually options to sit out certain activities, or is it all or nothing? I don’t want to book something and have her struggling every day. Some tours seem to have “comfort” or “easy pace” versions, are those actually easier or just marketing?