If you have looked at any of the RS tours on this website, you may have already seen this (slightly edited) under activity level:
Our tours are physically active! It’s an essential part of the Rick Steves tour experience. On our (fill in the name) tour — among other things — you'll need to happily...
Carry/roll your luggage over uneven pavement (several blocks possible) and up stairways to reach your hotel; then up several flights of stairs to reach your room.
Be on your feet, walking and standing for up to three hours, indoors and outdoors, in all weather conditions.
Sleep with street noise and no (or weak) air conditioning.
After orientation and transportation lessons, be able to navigate through the city on your own.
And you have probably seen this under itinerary:
Daily Activity Key
Light = 2-4 miles of mostly level walking throughout the day.
Moderate = 2-6 miles walking throughout the day with some hills and stairs.
Strenuous = 2-8 miles walking throughout the day with lots of hills, stairs and uneven terrain.
Each day of the itinerary is clearly identified as to the activity level. Believe what they say. I have only been on the Istanbul week tour and the distances were correct according to the person in our group who was wearing a pedometer.
Also believe those who mention going up and down steep terrain and steps and walking on cobblestones and rough terrain, even in cities. And don't discount the standing and listening to what the guide has to say. Sometimes that can be more tiring than the walking.
I cannot emphasize enough how important the right shoes are for being on your feet several hours a day with few opportunities to sit and rest. I noticed on our Istanbul tour 2 women who had apparently worn the wrong shoes one day. I think one had on flats, but I know one was wearing Birkenstocks. Both had slipped their feet out of their shoes and were standing on the cool floor of a courtyard at Topkapi Palace.
The right shoes and socks for you (there are lots of threads about this topic) will make all the difference in your level of fatigue, from foot to head. Training will also make a big difference.
BTW, my husband and I were 66 and 68 respectively last fall when we went on the RS Istanbul week tour. We followed it up with 4+ weeks in Greece where we visited places with similar terrain/steps/walking/standing challenges, but we didn't spend as many hours doing it each day. He did no advance training at all and I did less than I should have (only walk the dog everyday), but we both kept up in Istanbul.