Just a little more advice you may or may not want: John has pretty well covered getting up the mountain. There are only a few spots on the mountain that are downhill as you do the ascent portion of the race. For the 13+ mile ascent, you will encounter less about 1/2 mile of downhill running. So it's pretty much ALL uphill. But it's key to remember these downhill portions when you make the return trip. They may seem negligible as you run up, but 20= miles into the race approaching Bob's road, it may feel like you're doing the Incline (A wonderful, really steep trail in Manitou). And with 4 miles left in the race, just past No Name Creek coming down is a nice little 100 yard uphill that just might suck the life out of you.
The good news about running down Pikes Peak: All that O2 you lost as you busted your ass to get to the top comes back. You may well feel a lot more energy as you get down to Barr Camp, and travel down the trail. The key here is to make sure you stay focused on the 10-20 feet in front of you. The huge roots don't knock you down; it's the rock that protrudes 1/2 inch out of the trail that will knock you on your ass. The goal here is to stay vertical.
A sub 2 hour descent is an excellent time and given you Marathon time, I am guessing you could do this. But don't be disappointed if you don't. Bombing down Barr at 6-6:30 pace is extremely difficult even in the flatter areas. Tired legs from climbing and overall fatigue will likely make you slower. An overall 9:00 pace downhill will get you a sub 2:00:00 downhill. That doesn't sound fast, but it is.
Finally, remember that this is an out & back so you get to encounter every single person who is running the race. You appear to be fast enough that you won't encounter too much downhill traffic coming up that last 3 miles. But you will definitely hit the huge bubble of mid-packers coming back down those same 3 miles. This part of the trail is rocky in lots of places, and you have to avoid people who look like s**t who are 2-4 miles behind you. They are supposed to avoid you, but they may not (see the FOCUS thing I discussed earlier). Don't be afraid to yell out that you are coming so they know. These first 3 miles of the descent can easily take 35 minutes because of the traffic and technicality.