Two great links offered already and I'll probably just be repeating what they say in there. I'll answer one question and comment on one of your comments directly, though:
Is it more miles or more intensity?
During the summer, more miles. As I'm sure you're aware, there's only so long you can pound out high intensity before peaking. Knowing the D3 schedule, your top runners especially have to be looking for a late October or November peak. You don't want to be pounding out a ton of intensity in July. If you check out the links already offered, there is some intensity included to balance out the miles but the focus is on establishing that aerobic base.
Also, I might start introducing power lifting this year to build explosiveness in the legs.
I don't know too many runners who do power lifting. Personally, while anyone who has seen me race can tell you that I have next to no explosiveness, there was one season when my kick was at least average (which is a big accomplishment for a 32 minute 10K runner who never broke 60 in a 400). The one thing I did differently that year than any other was nearly daily strides at the end of all of my runs during the summer.
One final thing. Sue hits the nail on the head with the biggest challenge you're going to face with your top runners, especially the young lady. Talk to them as soon as track season is over to get the idea in their heads to go after those who are ahead of them in races. If they are near the top in the conference, plant the seed for goals of all-conference or a conference championship, depending on what's in reach. Get them thinking beyond being the #1 runner on the team for which they are already that. Give them bigger goals to shoot for that will keep them moving forward and not just trying to maintain their top spots on the team.