I'm guessing that what you're feeling are muscles being used anewly, or at least more intensely than before. When I first began using my big toe I was incredibly sore on the inside of the foot, but that passed after about a week. I make sure to use the big toe properly now when walking as well, not just running. Somehow walking this way affects my physical balance but also my emotional one, and my confidence level in general. Holy crap -7°C?! That is arctic. I'm struggling with 8°C, though my general vitality is still low, it'll take me time to build that I guess. Let me know how things go on with your big toe and lower calf? Thanks for sharing by the way.