I have not made myself clear or given enough information. This event was two days ago. I am fine now and was fine thirty minutes later. It was an actual chill.....I was freezing cold and shivering in spite of blankets.....though no fever or other symptoms. It was very sudden, about an hour after extreme exertion. I have no symptoms now. I am simply interested in figuring out why this happened.....and, of course, preventing any future episode.
I did understand you, and am not sure why you do not understand my reply, except that probably too many details that do not make sense if you do not have a background in biology and human physiology were added. I apologize for any confusion. What is important for you to understand is that, chilling such as you describe can occur after an intense period of exercise. It is a physiological response of the body resulting from a type of protein that is released into the blood stream during intense prolonged exercise of which the body is not accustomed.
To prevent it from occurring again, it is recommended that you do not take a hike of this length.
If you are interested in further hikes of this nature, it is recommended that you gradually build up your physical tolerance, over a period of months; or spread the hike out over several days.
As a health care provider it is of my nature to teach prevention, so I remind you, in case you are not aware, that it is also recommended, for everyone, to see their physician before starting a new exercise regimen or a higher level of physical activity. Another action that can benefit those that are planning on increasing their level of activity is to hire a certified fitness trainer (with doctor's approval). Some fitness trainers will work with a group of individuals at the same time, not only to make it more cost effective for the participants but also because some people enjoy the social interaction. If you are a member of a gym or fitness center, you may be able to obtain these types of services at no additional cost.
Being a backpacker and hiker myself, I know very well that 12 miles is a lot at any age!. But besides that, it was you your self that said it was a more difficult hike than usual. Also, you indicated that you did not understand what I was talking about, and this information is related to the immune system mechanics, and since I took physiology myself there has been a lot more discovered about how it works - so any one not reading this information and keeping up with it yearly might not understand all the details about how it works. So there should be no offense taken at any thing I have said. I will opt out though of your question and this will allow another expert to take a shot at it. It has been nice working on your topic, and I hope you continue to enjoy your active and healthy life!