i am also trying to discover the relative merits of different toughening approaches.
i think chip-n-seal is better than straight gravel because (plagiarizing from elsewhere) sparse gravel moves around under your feet and just hurts and slows without seeming to do much toughening. you can still run (very slowly) on the chip-n-seal rather than doing a prancing chicken imitation. a week or so ago, i did like 6 miles on varying grades of chip-n-seal at the beginning of a much longer run. yeah, that might have been a little more than was wise at this point: it took about 5 days before my soles were feeling as robust as before. i was still able to run and stuff, but they preferred to avoid the broken acorns/etc.... all told, i think there is merit in it for "toughening" for long distance and/or trail running.
sometimes i get these "zombie zones" on the front/landing part of my feet, especially in cold weather. they typically in the pad behind and between where the #2 and #3 toes are (i guess where there might be a little fold or crease during landing) and behind the #4 toe where a fair amount of weight gets supported . after my (so far) epic chip-n-seal run, they started making a re-appearance despite the warm weather. to see what would happen, i thought i would try a chemical treatment of some sort, in this case: soybean oil! that seemed to heal up the worn/thin skin and zombie zones, but it also (surprise, surprise) made the skin a teeny bit softer and more sensitive, at least initially. of course, that was kinda the point. i'm keeping at the oil treatment to see if it will help preserve the skin over the longer term and so far, it seems to be working even after my longest non-race run to date.
i've had mixed results with heat treating in the past. there's nothing quite as great as blacktop in florida in july; but, where i live we don't get the heat consistently enough to get a good enough effect on the road. my fallback (and winter) method is sand in an electric skillet with the aforementioned mixed results. but, my next planned experiment will be to combine the food oil with the heat treating to see if deep-fried feet
wrapped in bacon are delicious work better.
and then back to the chip-n-seal.
and then, perhaps, there will be some good conversation pieces: isn't there some cliche about "poor decisions make for better stories"?