Starting around the middle of training week #3, I felt hot
in the morning and tired during the day, and my right jaw
hurt when chewing. (上火 in traditional Chinese medicine,
I believe.) My resting heart rate stayed flat (51 to 53) 
and so I was not over-working the body. This year's flu
season passed and alergy was mild. As the symptoms
persisted, even fasting didn't fix the problem and I started
to blame caffeine.

I am not a heavy coffee drinker (about 12oz a day) but
nonetheless went through the 4th week without the beverage.
Withdrawal was worst on the second day. Then, I had three
cups during the weekend after running. They should but
didn't feel refreshing--so my problem was not simply coffee.
The following Mon, I had another cup and afterward felt so
horrible that I had to skip Tue's run.                                                                       

The same day, after much reflection, it finally dawned on me                                                 
that I was not drinking enough water. Before every run, I                                                    
would usually have 12oz water but that was often it. I
didn't drink during the 8- or 9-mile weekend runs as I
believed they were not long enough. The problem might not be
as bad had I replenished afterwards. But what I often did
post-run was to read and write or go through Tim's math at
Starbucks while sipping a cup of joe. Coffee makes things
worse as it is diuretic. How could I be so smart!?

So in the past five days, I kept a detailed log of how much
fluid I drink throughout the day. And the results were                                                       
elevated energy levels, receding hotness, and decreasing                                                     
resting heart rate (from low 50s to high 40s, although this is still higher than the previous average of 45).

I also did two "sweat tests" as detailed in a RunnersWorld
article and decided that I needed about 3 to 4oz of water
every mile when the temperature is in the low 50s.

I learnt that I need to have electrolytes, carbs and                                                         
proteins, too, to keep the energy and prevent muscle loss                                                    
and dehydration. So from a nutrition perspective, my first
marathon four years ago looked like a joke: I didn't bring a
bottle and started drinking water at aid stations only after 
the first 13 miles!