I live in central Ky. and for the most part, the winters are mild. Of late, I have been running in 25-25 degree weather. I am not sure if I am just getting better, but I have much more "easier" runs when it is cool out. It seems as if my breathing and stamina are much more in-tune during late fall and winter as oppose to the summer. In the summer, I automatically suck at least once or twice a week whereas the past month, I really haven't had much poor performance. Is this the theory of your body has to work harder to stay cool in the heat and works less in mild cold. I agree with the concept that in severe cold, it might take its toll. I am far from elite, but no slouch either. Good insight above though.