My penultimate self-supported weekly ultramarathon in 2008 took place in relatively cold and dry conditions. It was a nice day, but no luck with the sun as is almost always the case in the darkest period of the year. After all, Helsinki is as far North as Anchorage.
Anyway, I ran 46.5 km in 7:14 fartlek-style. No problems.
Now when it's 51 down and only the last one to go I'm starting to feel I can pull this off. I wasn't so sure when I began on January 1st. The first two months were definitely the hardest due to extremely cold and stormy weather. Now it's not a big deal anymore. I guess you can get used to almost anything, no matter how crazy it sounds initially.