Strenuous walking: 2–8 miles - why the large range?

Why not contact the Rick Steves Tour office and ask them?

I see that listed on some of the 'afternoons on your own' days...and though I can't speak about Barcelona, the suggestion of checking our Retiro Park in Madrid would easily add numerous miles to your day. It is big...and a distance from the museums to even get to the main entrance. So the range includes the 'recommended' way to spend your unsupervised time.

Why not contact the Rick Steves Tour office and ask them?

I wanted to hear from people who did the tours (any tour since it's in all of them, I think). And it's easier to create a thread than to call them.

Afternoons on our own make sense. The Toledo day tour and the last day dinner are also from 2-8, but maybe they included us walking on our own in Toledo.

I’ve been on 8 RS tours and the listed mileage is a guide only. For example, in Istanbul, our walking tour there had lots of hills, stairs and many steps as measured by my Fitbit. Other days on the Best of Turkey we didn’t cover as many miles especially on bus days. As someone already wrote, on your free time, you can choose to find a cafe, sit with a beverage and watch the world go by. On all my tours, there were days of strenuous walking sprinkled throughout. I haven’t been on your tour but I would guess both cities will have lots of steps.

I took the "Best of Barcelona and Madrid" tour in February. 2018. It is a genuine delight and I highly recommend it. Although there may be some adjustment to the walking itinerary, the description of the tour as posted elsewhere on this site is what we did five years ago.
Both Barcelona and Madrid are essentially flat cities. With the exception of riding the metro to the Avenue of Discord and Sagrada Familia, we walked everywhere we went in Barcelona until the morning of the final day, there when we took a bus on a sightseeing trip around the city. A little odd to do this on the morning of the final day, there. The bus took us to the train station (Sants) and from there it was high-speed AVE train to Madrid (Atocha) station.
Upon arrival in Madrid, a bus tour of the city was followed by check-in at the hotel - ours was on Gran Villa. The first full day in Madrid does have the most walking - from hotel to Plaza Mayor, Royal Palace, etc. Each day's touring ends prior to 14:00, at which time you are free to explore on your own. We walked all over Madrid on "Art Museum Day" - I recommend finding time to visit the Thyssen if you haven't exceeded your interest in art, Toledo was reached by bus - and Toledo is a little more hilly than either Barcelona or Madrid. Keep in mind one more thing - this is really a 6 day tour. The Meet and Greet at the hotel is in the afternoon followed by an included dinner. No actual "touring" other than a brief walk around from the hotel to the location of the dinner and the last day is over after breakfast. Still 6 days of pure delight - and this particular tour is hosted by several of Rick's absolute best guides. I hope you get the one that led our tour.
"Strenuous"? No, I would say "lengthy" and, at 68 (when I took this tour) I was in no better shape than you. We stopped frequently and enjoyed the magnificence of all three of these majestic cities.

Thanks for the replies. They're really helpful.

and they don't always have the same hotels.