+1. This was sort of my thought...I think some of the advice in this thread applies more to serious overland trekking than city tours.

I can't say that I've ever traveled with two pairs of walking/running shoes - even when I'm traveling to run a marathon. I alternate pairs when I'm running high miles at home, but definitely not for goofing around in a European city where I'm probably going to do 7-8 miles a day max, thanks to the fact that there's usually some other easy transportation options available.

There's definitely something to making sure you are in the right make/model of shoe for you. I can't recommend what exactly that shoe is for you because it's an incredibly personal experience getting to know shoes and settling in on the right one for you. I'd probably want to go into the trip with about 50 miles on the shoe, but honestly I don't think breaking in new shoes is quite as important as it once was. My preferred shoe, Brooks Adrenaline, runs pretty nicely right out of the box. I think Brooks also makes some walkers but I'm not familiar with them.

Go to a good walking/running store and get fitted by someone who knows what they're doing. I guess you should go to a doctor if you think you have really messed up feet, although I've never done it myself.

Serious hiking boots are total overkill for a city tour. That'd be like running the New York City Marathon in heavy-duty trail shoes. Talk about being knackered at the end of the day!! You want a lighter chassis made for the streets.

I think the best advice to avoid fatigue is simply to mix in an easy day here or there.