everything I can think of I'm sure you're doing. I was going to suggest logging your rides (you're on Bike journal)to get a sense of your progress and doing lot of climbing , although 6000 feet isn't a whole lot (I've done an imperial century with over 9000 feet) 6000 meters would a different matter.
Maybe Veronica or Nanci will wander by. I'm still a newbie when it comes to Ultra riding