I am 43, very overweight and unfit and have just come back from a 14 day holiday where we were regularly doing over 20,000 steps per day, every other day, with a day of around 10,000 steps in between.
My feet went through the mill a bit, due to me having awkwardly shaped feet and ill-fitting shoes, but I can't say that my legs ached at the end of the day. I was knackered, but my muscles didn't ache.
Now I'm not walking 20,000 steps a day I'm finding I ache a bit more and need a good stretch every so often.
Definitely try different, more supportive shoes as PP's have mentioned. If, for example, you're wearing crocs or something that doesn't completely hold your feet, you may be tensing muscles to avoid your shoes flinging off, which does hurt in the end.
I've got a few different pairs of Skechers shoes and trainers which are excellent for walking in (once they've worn in around my awkwardly shaped feet!).