Most everyone has heard of the Iditarod - the famous 1100 dog mushing race from Anchorage to Nome Alaska.  I recently returned from Alaska where I learned of the "Iditarod Trail Invitational", tagged the longest winter ultra race in the world.  You have two routes you can follow, a 350 mile and the original 1100 miler.  The main difference are the modes of transportation.  You can race on bike, snowshoe or on foot. And it's held in March where you it can be 30 degrees below zero with blistering winds.  There are sometimes days between rest cabins or villages so you have to carry a sleeping bag and food with you.  Visit the website here to learn more about it and read some of the competitor's blogs.

How about the "Great Divide Race" which follows the US Continental Divide for  2,490 miles of cycling?

How about a Deca IronMan with a 24 mile swim, 1120 mile bike and a 262 mile run?  Here's their website.

Having never attempted one but from what I can gather, finishing depends a great deal on your mental fortitude; how tired you feel, how exhausted you are, and how the cold and hot plays games with your mind.

Assuming a high level of fitness and training, part of these finishes might be determined by nutrition - have you practiced your nutritional in take on road and in all sorts of weather.  Which leads me to this excerpt reprinted with permission by Human Kinetics.  The book is "Endurance Sports Nutrition", by Suzanne Girard Eberle.

"The biggest danger with multiday rides, runs, treks and tours, cycling classics, sports camps, and climbing expeditions is incomplete recovery—