I ran 5:04 last week and ran 17:46 during cross country (on a hilly course), so I'd say it is fair to say 5 flat is worth 17:30 on a FLAT course.
hmmmm. I consider myself more of a strength based runner and I've ran only 3 miles this season:
however my pr during xc was only 18:47. I seem to struggle turning my track times into Xc times.