This question is really hard to answer because it varies so much based on running specific talent. Everything about a persons physiology dictates how fast they will be. Body type, natural vo2 max, muscle fiber type, durability and countless other physical attributes will eventually limit ones ability to get faster on a specific mileage. Also, one's athletic background prior to attmepting to run 70 mpw would play into it as well as how the 70 per week is structured. Are we talking 70 miles of easy running? Are they doing interval work, tempos, long runs etc. I'll throw my personal examples out there just for fun:
I ran about 70mpw on average during my senior year of HS and ran:
800: 2:00
mile: 4:20
3200:9:15
5000:14:57 (track)
6 miles: 30:41 (road).
Years later after a break I ran 70 mpw for an 8 month period as a 24 year old and ran:
mile: 4:25
5000: 15:08 (road)
6 mile: 30:32 (road -same road race as in hs)
12k: 38:53 (road)
half: 70:20
mar: 2:30