I had to chuckle, thinking about some of our experiences on RS tours. Our first RS tour was to Florence (please, RSE, bring that tour back!) After the initial group meeting, our guide Cecilia gathered us together outside the hotel, and said "Don't worry, I don't walk as fast as Rick does," and immediately took off like a shot, leaving most of us standing there with dropped jaws. We did manage to catch up, although we almost lost a few folks at a stoplight.
On our Belgium/Holland tour, we met a local guide in Delft. As she took off to lead us somewhere, our tour guide, Hans, called to her "Slow down! These are Americans!" And this after a number of our tour mates had been grumbling because they thought Hans walked too fast. (DH and I thought that tour was rather leisurely, perhaps the least active tour we've taken.)
I agree with Pam that the 21 Day BOE was the most strenuous. My personal bête noire is hills - I had to stop and rest a moment on the hike up to Neuschwanstein.
But as several people have mentioned, while you may walk up to 8 - 10 miles a day, it's piecemeal. And most days don't have that much walking - at least on the tours I've taken.
We do make sure we get out and walk as much as possible before a tour, and try to incorporate hills and stairs. Failing that, I found it very helpful last year to wear a daypack with 10 - 15 pounds of stuff in it on my practice walks. That helped get me in shape without having to add many more miles.
We have made a kind of vow, though: if it ever comes to the point that we can't keep up with the guide, it'll be time to quit taking RS tours. So far, so good!