A year before Pantilat, champion ultrarunners Hal Koerner and Mike Wolfe set the previous supported speed record on the JMT at 3d12h41m from Whitney Portal to Happy Isles (3d9h5m from Whitney summit) on August 1-4, 2013. A brief write-up on their trip is on the Runner's World site, and another article is here. Wolfe's personal account is on irunfar.com.
First of all congrats to Hal and Mike for an awesome run—I can’t wait to hear the details. There are those who question the logic and purpose of FKTs. I think pursuits such as these are very important. They inspire us and disrupt the ordinary. Had both of you failed this would have been a Monday just like any other Monday--a Monday quickly forgotten and therefore for all intents and purposes a Monday not lived. But instead you succeeded and today was special. Days like today remind me how wonderful it is to be alive and how we must constantly strive to truly live it to the fullest. Again congrats and thanks. -- Brett Maune