After wearing sandals for the last two weeks, I can't say it helped protect my callouses or that it made them tougher by keeping them dryer. However, I was able to run some distances, even on back-to-back days without causing any blisters or wearing the callouses down as much as before. They still wear, but I'm doing much better with my landing and takeoff. Last Wednesday evening I ran 10k--with shoes on--and my lower legs were strong enough for me to land just as when I run barefoot. No heel striking at all. So that was cool.