Endurance training is the basis of all my training, and takes up 60% of my training time. I probably spend half of my time Trail running or Mountainbiking, depending on the weather, where I am and how I feel. Both are great for endurance, and have a valuable cross over effect for skiing.
Strong legs for the winter
You can’t ski strong, without strong legs. Also strong legs can save you from injuries by getting you out of awkward positions that would otherwise end in a broken leg or ligament.