A couple of years back running on a  local college's track, I had tons of more energy to continue for an additional lap or two but was getting mildly thirsty, which registered as odd for me after drinking a glass of water 15-20 minutes before running.

The obvious hit me: Close my mouth. 

I was running and breathing with my mouth open--double whammy--drying out my mouth and not allowing myself to properly salivate. I was creating the illusion of being thirsty. From then on, nine times out of ten, I ran with my mouth closed for the entire duration. 

 A friend with whom I shared this "close your mouth" tip tried it--running with a closed mouth--and came back raving saying that while she could not be a distance runner, she ran a bit longer and more comfortably than when her mouth was opened.

I bring up this past revelation because last week, doing everything right--drinking 15 minutes before running and stretching to pass the time--it happened again: I got thirst, however, a bit differently.

"But I hadn't run yet," I thought.

Ephiphany number two: Chewing gum.

Another trick: Chew a Chiclet-sized gum like from Trident or Dentyne. This helps to promote salivating and get out on your feet.

There are tons of reasons folks--even those who hydrate well--can get dry mouth while running. The below article lists some of those potential causes.

You'll find out that not all of them are physical--psychological factors like fear and anxiety could trigger a dry-mouth-false-alarm.

What Causes Dry Mouth During Running? What Can Be Done?
http://www.livestrong.com/article/438274-what-causes-dry-mouth-during-running-what-can-be-done/