The marathon-training schedule is designed purely to enable our runner to stand up to the racing and speed training necessary in the eighteen weeks before the race that he has selected to win. The marathon-type training makes him capable of running long distances with ease, but he will have to go through a schedule of sharpening work before he can step on to the track and expect to run at a reasonably fast pace. So, for six weeks we will prepare him for that track debut. It is the hardest part of the whole schedule.
— Arthur Lydiard, 1962
The men in Lydiard’s day were a lot stronger physically than those of today…the schedule called for six continuous days of hill bounding and springing, followed by a 7th day of 20+ miles; for six-weeks.
6 days per week of hill bounding and springing for six weeks. Every 7th day was 20+ miles. Those were men who emerged from this six-week stint.
Lydiard goes on to say…As it appears in cold print, this schedule is a formidable and perhaps a soul-destroying one and I am always careful to emphasize to athletes that it is also a short-cut to exceptional improvement on the track. It is certainly hard work, but the only other way to achieve the same effects would be to work just about as hard over a much longer period.
These days I would not assign 36 days in a 6-week period of hill exercises, but I definitely would encourage a runner to go through hill training if they are looking for that exceptional improvement on the track.
Phillips Running 