From my experience, it seems that after a solid training block of 5 months, I take two weeks off, then start running again. By the 6th week back running I find I am just about as good as I was at the end of the last block, just off base pace running. Then over the next few months my fitness improves past where I was at the end of the last block.
I believe the lifetime miles allow you to easily reach your last fitness. The two weeks off is a recharge that allows you to train that next cycle without frying yourself.
I'm not even injury prone, it's really a matter of getting fried, if you know what I mean. Not mentally, the legs and energy systems simply get fried if you don't allow for some down time. And each time you're ready for a breakthrough shortly after the 2 weeks off.
I mean hell, these guys are running what, 120 miles a week? That fitness dies hard, and that stress needs a break.