I hope I don't across as too much of a jerk in this post, as I do think speed work is important; however, to say the OP should emphasize speed over endurance is bad advice. A key component of ultra training is figuring out how to keep going when something goes wrong and everything hurts. How is a person supposed to learn how to run on blown quads if the farthest he or she has run is 12 miles? How will someone know when to eat and drink or what foods and fluids even work?
Here's a bit on Matt Carpenter's training (
http://skyrunner.com/story/2005l3.htm):
"When he signed up for his first Leadville 100 in 2004, he had never run a race longer than 30 miles. He switched his training goal from running a marathon to running four marathons in a row. That meant two hours of running a day, no matter what.
Most ultramarathon runners focus on endurance by running as much as 50 miles on weekends. Carpenter chose to focus on speed. He never ran farther than 25 miles a day but did plenty of short, fast workouts. The training paid off.
In late June 2004, Carpenter ran the San Juan Solstice 50, a 50-mile race in the mountains around Lake City, as a test run for Leadville. He came in almost two hours ahead of the second-place runner and destroyed the record by 43 minutes."
The main things I take from this are that he was doing 2 hours or running a day (14-16 miles, maybe?) and he raced 50 miles. That's hardly what I would call "shorter training."
Again, I'm sorry if that comes across harsh. It's just that your advice flies in the face of everything that I have seen work for other runners, whether it's the fast people or the back-of-the-packers.