As implied by the poster above me about okie state, the long run is VERY important. I noticed the biggest change for me was getting good tempos in and good long runs. I adapted very well with positive changes when i started doing quality long runs, not just dawdling around at 710 pace.
Working in tempos during base phase is very important, notably for the mental aspect as well. XC 8k is a big mental battle, and you need much more strength than for XC 5k.
Make sure you're still doing striders after your easy runs, it'll make popping back into workouts much more pleasant.
Another good summer strengthener is find a route with some solid rolling hills and just run hard on the uphills, but not that you're gassed and bent over at the top, but that you can keep rolling.
Summer is all about just getting hella strong.