According to an article I have at home John Walker did about 75-80 miles per week in the base training phase at a solid pace, often the ten milers were in 50 minutes. He did this for 10 weeks before transitioning. I don't have much else to share other than the fact that he did longer intervals at round 4:40 to 4:20 pace before doing speedwork and then he didn't really do long speedwork sessions, but did them often as he neared racing season. I wish I had more. By the way, I think that John was 4th in the World Cross Country Championships one year. Great aerobic capacity and strength!