We've been on a couple of Rick Steves' tours, and no, we did not train ahead of time. But, we are very active, and really long walks, hill climbing, etc. does not phase us.
But, as Ken posted, the long periods of standing (relatively still) is something I noticed, and sometimes walking more slowly than I am used to (such as in a museum or going at a slower 'group pace' walking with a tour group) is something that will bother my back a tiny bit. When my husband and I travel on our own, we can go at a bit faster pace, and it is not as tiring.
But (seriously) since you are asking the question, if you have any concern whatsoever, do 'train' a bit. Do an extra mile or two of 'intense' walking each evening when you leave the office (or at lunchtime), and do some 'group strolling' with friends just to practice having to pace yourself with others.
.....and extra exercise you get in the process is likely good for you anyway. And, not knowing if your BMI is good/bad, etc., it is hard to advise.
A few times when we have taken cruises, we've seen one or two (usually over-weight) people on shore excursions who huffed and puffed after taking 3-4 steps to enter a bus. If someone is in that kind of physical shape, likely they would need A LOT of training for a RS tour. But, if you could participate in a local fundraiser 10-mile walk-a-thon and keep a reasonable walking pace (and mentally factor in some breaks every two miles or so....since the tours do not go 10 miles without stopping), then you would likely be fine.......just remember, the next day you would have to do at least half of a walk-a-thon, and ditto for several days after that.
But, generally the Rick Steves' tours are designed for the mass market, not athletes. But, I've never seen a person on a RS tour that was totally unfit. I think the disclaimers he gives causes people to 'self select' if they would fit in (or personally enjoy the activities).
Just the fact you are aware, you would likely do okay.....but, yes, maybe step up the walking a bit and include hills and rough surfaces. And, as other posters said, don't go on a tour with new shoes......wear comfortable, well-broken in shoes AND SOCKS!! And, put a different pair of shoes on for the evening or afternoon. Just changing shoes sometimes relaxes the feet.