I don't normally do 400 mile rides in cold weather, but 200-300 mile rides, often below freezing, yeah, done my share of those.
Heated grips (with proper hand protectors to keep the wind blast off) and an electric vest (I prefer the Aerostich Kanetsu, but there are plenty of good ones out there) made things a lot more pleasant. My general cold weather gear-up is: some kind of wicking thin polyrazzmatazz base layer (draw the sweat away from the body--it's strange, but even in single-digit temperatures I sweat some). Flannel-lined jeans on the legs. Electric vest underneath a big ol' Patagonia sweater (that I bought sometime in the '70s!) up top. Whole thing goes under my Roadcrafter two-piece. Most of the time, I can get away with three-season gloves thanks to the grip heaters and hand protection. Full-face helmet (obviously). Good neck wrap of some sort--I want to try one of those silk scarves, to see if it's really slippery enough to make turning my head easier. Heavy wool socks inside boots. I haven't had much luck with the little chemical boot heaters, so most of the time I don't bother with them.
The key thing is to plan more stops, and plan your stops around warming up. Remember that getting into and out of your gear at a stop will take longer (and you want to shed the gear when you go indoors to warm up, to avoid overheating and sweating).
Your temperature may vary, of course.