My (amateur) opinion is the 80% preferably needs to be running, as you are then building the necessary strength/conditioning to be able to sustain the higher-intensity sessions.

Although this may fly in the face of popular opinion.... a large number of clubmates attain a far higher level of ability than me, off a far lower weekly mileage.... i find though that my level of performance remains fairly constant, where others on lower mileage reach higher "highs" but don't always consistently perform.