Iron Cross ain't no joke. Ask around. It is fun and different and a big hoot - especially if you can laugh at yourself for carrying your bike up Wigwam, but it ain't easy. Figure 4-6 hours with tons of climbing, technical trail bits that will challenge your skills, fast gravel and pavement and some pretty decent off the bike work.
The best preparation you can do is mimic the demands of the race a couple times between now and the event. Spend the days you can't ride for 4-5 hours focused on recovery from the big rides and spend some other days developing that sustainable power for the big climbs and long day on the horizon.
A couple long rides on gravel roads with lots of climbing should be a part of the training package. Get used to the feel of the bike sliding around under you a bit on those downhills. You need to add in some long hill reps on other days to work on threshold power and some days of long tempo efforts for the hours you'll be pounding it out around the Michaux roads.
Getting the 'cross bike out on some technical trails and work your way up to difficult rocky descents since that's where things get most sketchball on a 'cross bike - and getting accustomed to how the bike handles technical terrain will prove very beneficial in your IC prep. Those of you who've done this thing know what Lippencote Tr. is. Those who haven't are likely about to ride the most technical descent they've tried on a 'cross bike. While most people end up walking down much of Lippencote what you should know is that the front guys rip it on their 'cross bikes sliding their bikes through the washed out rocky trough and bunny hopping the downed logs. They can do this because they practice doing it. You should get out and practice.
If you're really geared up for Iron Cross then you need to be running too. Don't believe me? One of the guys to win this thing was Rob Jebb, a multiple time 3 Peaks winner and a world high altitude marathon series champion. Where did he make his move to win Iron Cross? He RAN up WigWam. Let me say again, RAN up it. Most people barely move faster than plodding on that run-up. It is steep and rocky and then you hop back on the bike for a short stint before getting off to run another section that isn't quite as steep but is plenty loose and rocky making for poor footing. Another remount and final run section await before you top out. IF you can move fast through those running sections you can gain minutes - in that 10 min section Jebb gained more than 1 minute on the next rider. It is a crux of the Iron Cross and of prime importance if you're looking for your best possible result.
Have fun out there and get ready for the coolest and mostest funnest 'cross racing going. IC X.X.X.