For me it is incorporating a long run and a medium long run a week. Then get 3-4 runs at around an hour. Then, when in shape, turn 2 of those hour runs into hard workouts. If I feel I need additional time on my feet then I do so. Whatever the mileage is, it is. When real tired I rest.
I primarily run base through November – March thereabouts. Take it easy the last half of May and spend June – July just running with an occasional race.