"As the climate changes, Mongolia is one of those countries that will go from being a water-stressed country to a water-scarce country," said Isaacs.
Sunila Jayaraj, 29, never ran a mile in his life until he discovered "pledge runs" as a way to raise money for victims of his native India's devastating drought and flood cycles. Born in the semi-arid region of Kolar, Jayaraj is acutely aware of the devastating impact that lack of clean water can wreak on a country as densely populated as India.
"The population itself is a big crisis, and the water and the population make it worse," said Jayaraj. "Education and health -- everything is linked to the water."
But it wasn't all hardship. Japan's Taeko Terauchi and Canada's Jason Loutit started dating in France and got engaged in Russia.
When they made it to Japan, the couple stopped long enough to receive her parents' blessing. They then got married while running past Niagara Falls.
For them, the phrase "long-distance relationship" took on a whole new meaning.
The newlyweds and the rest of the runners are now in the home stretch. They've made it across most of the United States. Their excellent and very tiring adventure ends this Tuesday back where they started in New York.
ABC News' Mike Cerre and Suzanne Yeo contributed to this report.