Let it Rupp wrote:
Doing "double threshold" is just a convenient and efficient tactic to adding more volume/intensity to your workload. The only people who would even need to double threshold workouts are those who have maxed out their mileage in single workouts.
That's fine if by "need" you really mean "need". A lot of posters conflate "You only NEED doubles at xx mileage" with "You SHOULDN'T double until xx mileage. More runners would benefit from more doubles and from double threshold; not because it's the bleeding edge of difficulty, but because it is actually easier to accomplish each session and easier to recover.