That's what I was finding Nigel. Towards the end of some of my long runs I was getting fatigued and maybe not concentrating quite as well.
At the end of the day, I'm not sure what being able to do three or four hundred miles a week is really proving? Once you can do it once to show yourself that you can manage it if you have to,
after that I just feel you are overtaxing your body needlessly. Pretty sure one long ride a week is enough to keep the endurance side of things up.
Oh, and as a bonus, since I am on a very limited income there won't be as much wear and tear or the bikes which is always a good thing.