Exactly and we have touched on this here previously — more than once or twice, incidentally. The article that r-at-work posts from Magness is one of the best I have seen on the topic, laying down physiological justification and implication for doubling (or tripling). The consistency principle at work. The reason why the Hansons marathon plan for low mileage runners (despite the lack of any run longer than 16 miles) and their high mileage pros works so consistently well — and why training plans that call for days off before and/or after a long run just to force runs too long (confidence is no substitute for fitness) for that level of volume into the plan so often leave their adherents underprepared for their racing goals. Having 'fresh legs' for workouts is not the goal of training, preparing specifically for the demands of the race is what should be the goal. Additionally, one should be mindful of the purpose of the run, where it fits into those race preparations, and stay present in that purpose during the run. Time and place.