Back when I was into cycling alot, I was told to belly breath not chest. Thats why the pro road bikers look like they have big bellys.(thats not why mine looks big lol)
So I noticed yesterday during running Zone 4 LTHR (170bpm) I was breathing in--4 strides, out--2 strides.
And if ya keep sipping from your camelback that helps the drymouth thing too--and it's more efficient hydration too(less energy to process, and better absorbtion into tissues)
To help with the cotton mouth I'll stick a piece of gum in my mouth, don't chew it though. It just goes to the side of my mouth to get the juices (saliva) flowing.