Look. Just to help the original OP out a bit, I have GOT to add in my two cents.
I trained for two years every day with a sponsored athletic group. We had PR's for guys that ranged from 1:46 in the 800m to 1:03 in the half-marathon, and at one point had 5 guys that had gone under 4 min in the mile.
6 minutes per mile was not an everyday thing. During long runs we would average over that, and steady state runs were somewhere in the 5:15-5:30 range, never exceeding 10 miles.
So for your co-worker who claims that he and his military buddies used to run "6 minute miles all the time", he is just plain full of s***. You can laugh in his pathetic face and walk away with your head held high. Seriously, that pace is "easy" only in races. In every day training, 6 min/mile never quite feels comfortable, even for athletes that run under 5 min pace in long distance races. Joe-blow is not going to be able to maintain that pace easily as part of his daily workout.
And by the way, 18 min for 5k, for a HIGH SCHOOl GIRL is damn impressive. Any college coach outside of the top 5 DI would recruit her, end of story.
Your co-worker either does not remember his old training times, or recalls one or two PT sessions where he cracked 7 min for a mile, and tells himself that was an everyday occurrence. He is a joke.