Your pace will (should) vary from switchback to switchback as the grade changes. Also as you go up there is less oxygen so your pace will naturally slow way down. The goal should be to manage a constant effort for the trip up.
For the uphill, I'd pick a target heart rate and use that as a guide. As a starting point I'd pick the same target as your marathon. For the first half of the ascent portion (up to Barr Camp), your goal should be to not exceed it. For the 2nd half of the ascent the goal will be to try and keep it up that high. :-)
For the downhill, go as fast as your legs will let you. From a hiking perspective it is not very technical (class 1). From a running perspective it is very technical, especially in the shady areas where it is hard to see where all of the rocks are. To give you an idea, the fastest runners are typically in the mid 1:20's for the 12.88 mile segment. Sub 1:20 decent times are few and far in-between and only 4 folks have ever done it (the last one being in '97).
If you want some splits to use as a guide, take your marathon time and add 15 to 30 minutes or so. Use this as an estimate for your ascent time. Then look at http://www.skyrunner.com/search/find.asp?Year=2013&Race=M&DNFs=No&Blanks=No&Sort=Asc to see how other folks who got to the top in about that time did for splits on the way up and then back down again.