My belief (for 25-40 year old hobby jogger) for 5k times:
10 miles - 30:00. If you can't break 30 minutes when running 10 miles a week, drop some mileage and spend the time on something else.
20 miles - 24:00
30 miles - 20:00
40 miles - 18:00. There is no reason to be running 40+ miles a week if you can't break 18 in the 5k when doing so. Spend some time doing something else.
50 miles - 17:00
60 miles - 16:00
70 miles - 15:15
80 miles - 14:50
90 miles - 14:25
100 miles - 14:00
What do you think?