It is fast for sure. 21 miles of a gentle downhill helps to run about 20 seconds below your normal pace. The scenery is beyond beautiful and the volunteers and officials did a great job to make this race one of my favorite ones. The elevation is not an issue, as you come down significantly from the start to the finish line. I even felt like I had a second wind around mile 20, since the oxygen was a little richer.
My only suggestion will be to either start the half marathon sooner, maybe around 6:00am so that the fast marathoners do not have to fight over water in the water stations with the slower half marathoners who tend to be walking around the water stations or even stopping next to them; or to make the water stations a lot longer. Currently there are only two tables per station, one for water and one for gatorade, maybe it should be 6 tables long, so fast marathoners do not get the traffic.
For example, a marathoner than finishes in less than 3:15:00, hits a very large traffic of half marathoners starting at mile 20 or so.
Again, a simple starting time adjustment for the half will solve this issue, or making the water stations a bit wider.