kdkdk, I coach an elite HS boy (14:41 5k, 6 state champs, 2nd overall at Footlocker Regional) who does most of his running at close to 7:00 pace. It works for him and that is where he is comfortable. I am able to run with him most days, even at my advanced age (49). Some days he pushes it down to 6:30 or even 6:15 pace if he is feeling good, but not usually. He does run a lot of miles, though, peaking with several weeks of around 85 miles in the preseason build-ups.
When it comes to workouts, it's another story. He can handle big lactate threshold workouts at about 5:12 per mile and long VO2 max intervals at around :70 per 400m. He is very consistent in workouts, which I think is partly attributable to his moderate pace on recovery / easy days. His racing is also very consistent, barring the occasional illness.
The point is that the pace of general mileage is an individual matter. They need to feel pretty comfortable and that means different things to different people. I'm convinced that many athletes of all ages run them a good bit faster than they should. Does Mary Cain run them too fast? I have no idea, because I'm not her coach watching her run every day.