You could add some other guys like Jonathan Johnson doing about 20mpw (and 1:44 off that mileage). But 400/800 is 400-based training. It is not about mileage. The "mileage" done is mostly in workouts. The endurance comes from things like special endurance 600s a little slower than race pace, much like the endurance in 400 comes from special endurance 300s and over distance 450-600. It's not uncommon for 400/800 guys doing more tempo-based (sprinting intensive tempo @80-90%, not distance-based AT tempo) training, or 3-4 workouts a week.