As a runner in my 30s with hardly any talent, running 70 miles per week with a couple days of intervals brought me to <18:00 5k without any struggle (I wish I had taken it further).
I don't think 70 miles per week should be the standard for high school (maybe summer before senior year if you're a sub 5 miler), but 70 miles per week in college or as an adult, with some smart interval workouts, will accomplish a lot.