Advice about how to train for an Ultra

First, I'll say I have not done an ultra.  But, I think I will this year.  Here's a sight loaded with advice from some great ultrarunners:

http://www.ultrunr.com/

On top of that, Hal Higdon has a 24 week ultra plan, designed to prepare you for Comrades, but I'm sure you could fiddle with it and make it fit.  Here it is:

 http://www.halhigdon.com/ultramarathon/ultramarathon2000.htm

Also, I would look at Runner's World Online, and check the Ultramarathon and Trail Runner Forums.  Lots of experience and advice to be had there. 

What distance are you targeting?  Are you currently doing any single track/trail running?  IMO, the most important aspect to an ultra (assuming you are choosing a trail one) is training your body to run trails.  This type of running is significantly different because of the lateral movements, the leg lifting requirements, and tricky foot placement.  However, I also think it is 100 times more fun than road racing because it engages your brain continuously.

I have done a 50K, 40 mile, and 50 mile race...and am registered for a 100 mile race in March.  There is also a group of us that are running the Tahoe Triple in September (3 marys in 3 days around Lake Tahoe).  They have a 72 mile ultra event as well...but the 3 in 3 seemed more fun

For the shorter races, my training was similar to marathon training with a few extra 20+ milers thrown in.  However, for the 50 and 100 mile races, I am using a plan that has two LSD runs back-to-back on the weekends (using alternating weekends to give my body a break).  For example, during peak training, I will run 25-35 miles on Saturday and then another 20-25 miles on Sunday (or vice versa).  The ability to run those long runs with minimal recovery time seems to be the cornerstone of most ultra training plans.  Right now, my biggest issue is night running--it is weird because your senses are all distorted.

Hands down, the most fun race was Dances with Dirt 50K (they also have a 50 mile race).  It is more adventure racing, river walking, and dirt ladder climbing than true single track running.  The Triple Lakes 40 mile race in Greensboro was the hardest race I have done, because of its technical trails and the red clay surface, which killed my joints.  Sunmart 50 miler was fun because of its large size and cool race swag, but it was the most mentally challenging because there are really fast runners competing for prize money, so you feel like a slug at the back of the pack   In any event, if you can run a race with pals, it makes the time fly by...but your abs might hurt from too much laughing.  My pals Trixie and Q (and TriGods for Sunmart) make any event one helluva experience!

Let me know if you have any other questions on ultras.  I don't claim to know much and I make sh*t up as I go along...but I do love them and am always looking to run more!

Thanks for all the good advice.  I'll definitely look at the websites mentioned.  I don't know what got into my head to do an ultra.  What prompted you to go the extra distance?