Does anyone have a feel for how long it typically takes to get completely accustomed to zero-drop shoes?

I bought a pair of Altra Lone Peaks back at REI's anniversary sale and have been walking about five miles a day, 5 or 6 days a week, with some uphill, and thought I was getting used to them.
Well, I did the Gnarl Ridge/Elk Meadows loop on Mt. Hood the other day(maybe 9 miles and 1500ft elevation gain) and to my surprise, my calves got pretty sore, and are still sore two days later.

Usually, if I've been walking regularly, I can do more than that without getting sore. Of course, it was more up & down than I'm used to, and I'm getting older as well.

I'm just trying to figure out how much of this I can attribute to the shoes. On the plus side, I had no foot discomfort at all.