Giles, Mu, Hodgkinson, Brazier are all 800m runners with the occasional 1500m foray, so they don't need much mileage. Valby is not ever going to be an 800m runner and is not likely to make an imprint on the world stage at the 1500m either, so she needs mileage or the equivalent of mileage. I would agree that she won't reach her ceiling without being able to tolerate a lot more mileage but she gets a great deal of aerobic base already from two hours of arc trainer/swimming/cycling every day, and that is as much aerobic work as any runner below the marathon is getting. Moreover, she is not at all capped in potential on her current regimen right now, because she closed that 14:56 in 2:53 (14:25 pace), looking fresh and with no 14:40s goal. With higher level runners, she would be in the 14:40s already.