I think you might get SOME
I think you might get SOME conditioning...but not much...in my 7 mile trail run with them this week, there def. would have been places I would have gotten several stone bruises on my arch, which I don't mind at all, but didn't due to the VFFs....and had absolutely NO tenderness....
conversely, yesterday I ran 5.5 total road miles, one of which was on new chipseal, bare, and still feel it a little today :-D
But since that was my only VFF run to date, I don't know how long I'd have to run on them to "feel" it later....the trail I ran ends with a mile of that 2a stuff.