So wearing appropriate gear while your outside can have a huge effect on how your training session will go. I took these photos this morning before I went running. You would think that you would become warm enough not to need all that clothing, but that isn't always the case. This morning it was between 30-40 degrees. My body would burn just as many calories trying to stay warm as it would running! I can tell you why this isn't a good thing....
Of all the energy pathways your body can take, the one it utilizes the most during aerobic exercise, takes the longest to get going. This oxidative pathway uses fatty acids as it's primary source of energy. Fat burning (beta oxidation) is one of the longest chemical reactions in your body. Fat burning is more efficient if carbohydrates are being metabolized at the same time. This is why you shouldn't go exercise on an empty stomach. The metabolized carbs keep your body from turing to it's other energy source while it is waiting on the fatty acids, muscle. That is right, your body will break down the muscles WHILE YOU ARE USING THEM to make energy. So you must maintain a delicate balance while doing aerobic exercise for any length of time. You don't want that hard earned muscle to be catabolized because your body is too impatient to wait for your fatty acids to break down.
You don't have to eat a lot to get this system in gear. I ate a slice of bread right before my run, which was 2.33 miles. Running 6 miles though? You might want to bring another slice of bread with you!
All these things are important considerations for people who walk for fat loss as well. Walking is aerobic exercise, just not as intense as running.
So if you get too cold, and your body has to use your carbs to stay warm, and gets too impatient to wait for your fatty acids, it will turn on your muscle. So stay warm everyone!