It's true, I actually spent a good 15 minutes of my life doing some Google-fu on the length of the Mekong and researching it's source. Ultimately, the length of the Mekong is 2,703 miles which is far too many miles for me to run in 2014. :( Additionally, my OCD tendencies started to twitch uncontrollably when I couldn't find sufficient data to locate a specific source of the Mekong. I found that "by international geographic convention, the geographic source of a river is the spring or glacial discharge within the drainage basin which is furthest from the mouth of the river." For the Mekong, this glacial discharge area is called Lasagongma Spring in Tibet. Here it is:
Please keep in mind there is still some debate about Lasagongma Spring as the Mekong source, but, what the hell, it's the best I could do with 15 minutes and a MacBook Pro based in Salt Lake City. I'm not sure why I thought I would find a specific source of a huge river. It's not like there's a giant river gnome sitting atop a hill somewhere dumping out an unending bottle of Aquafina to build the world's 12th largest river. . .