As John Kellogg would say, 70 mpw in high school is not high mileage, it is moderate mileage.
Here is a good quote from him on this subject:
"People cite examples of a runner here or a runner there in order to make an attempt at "proving" a point.
However, when hundreds of U.S. runners of many different performance levels are scrutinized, the stats
are overwhelmingly indisputable: High mileage in high school results in far better performances in high
school and somewhat better performances at the peak of a career. But moderate regular mileage (50-70
mpw) in high school supplemented with sporadic exposure to high mileage (15+ mile days) during that
time, followed by regular high mileage in adulthood, results in the best overall career performances."
In other words, Sara is right.