One of the things you need to realize is that an 18-year-old freshman is not capable (on average) of what a 22 year old senior or a 23-year-old red shirt senior is.
That said an average (by letsrun standards) d1 team probably would do an 18 miler around 6:20-6:45 pace assuming it is flat, this would be a hard effort but they should still be able to wake up the next day and go for an easy morning run (same pace, but maybe 7-8 miles)
A hard, almost all out effort, would be around 5:30-5:45 pace--most wouldn't be able to run more than a few miles the day after this.
People seemed to agree with my suggestion that low 20s is probably the max for college, so others would probably agree with me on this that if you're an incoming highschool junior or senior the longest run you could possibly need is about 15-16 miles, you are less physically mature than college runners and you're training for a shorter race. The pace for something like this if you are an elite runner (9:20 2 mile or better) would probably be at ~6:50 for a medium/hard effort, and 6:20-30 for a killer run.