I'll bite.
With no fast running (I'm going to assume no strides, hills either in reference to the previous poster):
I'd say just under 19:00.
But I would add that I don't think it makes too much difference whether he runs 100 mpw for 5 years or 50 mpw. More important factors would be:
1. Longest single run every two weeks
2. Frequency of runs (every 24 hours, every 12 hours, every 48 hours?)
Your body operates on a daily cycle, not a weekly one. Even with purely easy running, you are still training it to be stressed, and then adapt to that stress. That very simple idea of what fitness is has little to do with a total number of miles at the end of a week.