I agree with those who are suggesting that you need to diagnose the cause of the fussiness. Is it a specific issue, like plantar fasciitis, or a bunion rubbing, or arthritis, or low arches...or is it not being used to walking on hard pavements, or not being used to walking in general?
If it's the latter, working up to walking those distances will help. My husband does not walk as much as I do in daily life and his feet are always much more sore than mine after a long day of travel -- yet there is nothing technically "wrong" with his feet.
If it's a specific foot issue, then you'll need shoes that work for your feet.
If you wear Superfeet, make sure you're wearing the correct version. And you might need a different insole in different shoes. I wear one in my white Eccos and a different one in my silver Eccos. Partly because that's what will work in each shoe. The lower white shoe won't accommodate the insert I really need for my higher arch, but at least will accommodate a thinner one.
I have narrow heels, plantar fasciitis, a higher arch and instep. I walk (or sometimes skate or jog) an average of 5 miles on an ordinary day in my normal life, and 10 or more miles each day of city travel.
Plus, I hate to compromise on style! Yikes.
The only thing that works for me is options. I change my shoes a couple of times a day. If I can, I take more than a carry one. If I have to take a carry on, I will forego other things so I can fit more footwear. Or shove an extra pair of shoes/ sandals in a tote. This is all easier in summer, of course, when they squish down more than a boots do.
Booties/ boots are best for travel but obviously not in the summer. Below are options that have worked for me in combination -- never completely alone. The Fitflop is good for the PF. Most styles are too wide for me but this one buckles tighter so it works okay and the cushioning is great. I wear Superfeet in both sneakers and always wear little Smartwool sockettes as well with those. The Sofft sandals were great (but they don't make these similar styles any more and the more recent versions don't work at all as well for me. I still mourn these styles!) The Franco Sarto was a surprisingly comfortable sandal for long walks because the suede was really soft and the heel stable. I'd wear those if we were going out for dinner or something on a trip.