I never ran a mile slower than 6:30 in college, well maybe a few. Was a numerous x all-american and several times was in contention for the win. I never once felt like I was pushing on the runs. Most normal runs were 6:10 pace and long runs would just naturally end up at around 5:50 pace. These were hilly runs. Post collegiately, the runs were even faster, with many runs dropping into the mid 5's for the majority of the run. Represent the US and raced in trial races, US champs, etc.. As long as you can hold a conversation, you are running easy enough.
Some might like to go really easy, 7 min or slower, to each their own.
With all that being said, I would think anyone who can run 15:00 - 15:30 should be able to handle sub 6:30 pace without any strain. The only way it would be hard is if they are running their workouts too hard, as mentioned by another person on this thread. Often times, workouts are run much too hard because a majority of the coaches don't know what they are doing or are just lazy.
From all my experience, I will say that you can have success by running your easy days slower than needed, as well. There are many ways to skin a cat. On the flip side, to say that running 6:30 pace is asking too much for a mildly competitive college team is rubbish.