We do something similar at our HS. During the base phase, we focus on pace since we don't have "hard" workouts. The girls try to hit base mileage close to 90s over their mile date pace. A 5:30 miler would run close to 7 min/mile. For a Mary Cain, this would be closer to 6:00 pace assuming no workouts are being done. The reason why we do this is to become better aerobically during our off-seasons. This pushes the pace a bit (I call it "focused pace") but it is slower than they can handle cruise reps or tempo runs.
My gut feeling is that she is doing some training for a big indoor meet, so her easier days are probably closer to 2 min over mile time. That would put her near 6:30.
I don't think it is unreasonable. The Texas elite girls typically run mid-6s on daily runs and they produce 17-mid 5k XC times and they could probably run 16-something on a track.
Other point....
Regarding base mileage being relatively faster for women with equal PRs to men....
I think this is the case because of how they have to arrive at that PR. Think about it - two 15:00 5k runners, one man and one woman. The man is likely better in the 400, 800, and mile off of genetics alone. So the woman had to arrive at that PR due to superior aerobic development. What reflects aerobic development? Daily pace is part of that. Just a thought.