Reviewing the 2013 and 2012 confirmation lists (http://www.pikespeakmarathon.org/regstats/2013/ppm_qtime.htm and http://www.pikespeakmarathon.org/regstats/2012/ppm_qtime.htm) may be of help. It shows the results sorted by qualification time.
As you noted, there is a huge variation in the times. I know that in a lot of the local flat races, I can keep up with folks who otherwise crush me on the ascent. On the flip side I've also seen folks who can pull off a sub 2:30 marathon totally blow up in the first 4 miles and DNF. Running uphill engages a different set of muscles, so not working on them ahead of time will result in issues. Your 15% dreadmill workouts are a very good start (and comprise a lot of my training nowadays, and I'm local).
Downhill speed comes down to technique, technique, and leg turnover speed. (Combined with the ability to not drag your feet and thus avoid taking a potentially very nasty digger.)
As a first timer, all I can say is to go out conservatively and save your energy for the top half. Most folks seem to go out way too fast and end up in a death march above treeline. Speaking of which, remember that in many cases, it may be faster to simply walk than run. I've walked right past a lot of runners in the past. It drives them a bit nuts. :-)
Other than that, have fun.