To start building endurance in your core, practice these exercises daily a week or two before you move into lower extremity strengthening and plyometrics, and then keep them in your regular routine.
From there you can move on to the traditional ski and snowboard conditioning exercises such as squats, jump squats, box jumps, agility drills, etc.. I also recommend incorporating high intensity interval training (HIIT) into your workouts to really boost your fitness level. More on HIIT to come in my next blog. Happy winter!