I've wondered about this too. 3x/week I take a cardio/strength class that hits all the muscle groups while using weights, and also develops agility and balance. In my fairly recent foray into trail running I really feel the value of those workouts, especially core strength, agility and balance. The classes are why I now prefer trail running over road running. I can employ much more than my legs for a more engaging and satisfying run.
I don't expect Eric to give away training program details, but I'm having difficulty envisioning what it entails and what other training is acceptable without undermining the run.