As mentioned above, warmth is key. You're going to somewhat to really wet regardless.
This year (Jan 2014 onwards) for my single day mountain bike rides (3 1/2 to 5 hour) I've been using a combination of softshell tops and bottoms as my main rain layers combined with wool and synthetic layers underneath. This is experimentation. ***This is not at all my recommendation for TD*** It works for me for up to about 5 hours before it soaks almost all the way through and I start getting too uncomfortably cold. And by uncomfortably cold I mean in the shower for 30 minutes post ride to get my feet and hands feeling somewhat normal again. But while moving in the trails I'm good. So it's applicable in this situation.
For touring or bikepacking I bring a shell jacket, shell pants or knickers, rocky goretex socks and myog cuben waterproof lobster mitts that go over wool duragloves. This in addition to my sleep system and other packed layers and insulation keeps me confidently dry and safe feeling in rain/wet down to 33F. Plus proving to myself that I won't die when I ride in cold rain in February without shells gives me the confidence, feeling and knowledge of self to handle being wet in the mountains or foreign places with shells.
I just got a new OR helium 2 which I'll throw in my jersey pocket to test out hopefully it will prove to be more breathable than the northface diad it replaced. Driducks are neat too for certain applications but wet out eventually. Most breathable rain gear I've ever used though.
I've never done TD but have been a fanboy since before the DVD
I have toured and bikepacked quite a bit all over the world though. For me, I can't sleep when my feet are super cold, so I do things to make sure that before bed they're reasonably warm otherwise I'm wasting my time lying down. While I hate the feeling of my hands being so cold they're numb up to my elbows I can still function and ride like this so they're less of a concern for me.
You've got to ask....
Will you put your down jacket on between your sweaty body and soaked from the inside rain jacket when you start to get cold? How will that play on the loft of the jacket?
Will you climb into your bivy and sleeping bag wearing your wet rain gear because you're wearing all your layers and are too cold, tired, lazy to take off the rain shells? how will that play on the loft of your down bag after a day or two?
What if the north sections this year are really wet and sloppy unlike last years dry and fast conditions, can you go from riding all day in the piss to sleeping in it? what shelter are you using?
Marshall's summary is pretty spot on and having completed TD and a bunch of the other big races I think more than many he knows what he's talking about.
I would add that for something like TD where volume and weight are crucial over 2700+ miles your rain gear selection has to be looked at as part of your overall system; in combination with your insulation, additional layers and sleep system not independent of. The type of insulation (down, synthetic) should be taken into account with the type of rain jacket you'll be using, with the type of sleeping bag, bivy, tent, you'll be using.
I hope that was somewhat helpful or if anything gives you something to think about.
Best of luck in the big race, I'll be following along as I always do. Super stoked on everyone getting ready!
