There are few triathlon and running coaches out there with more knowledge and experience than Tim “Lucho” Waggoner. His coaching tips are pure gold (Hint: listen to the Endurance Planet podcast!).
He talks a lot about is this ongoing tension between quality and quantity in training. Boiled down to one sentence, which paraphrases the advice of elite running coach Brad Hudson, is to:
“Never neglect volume for speed or speed for volume.”
A pretty simple principle that should guide every endurance athlete’s training.