There are as many opinions about weight training for ultrarunners as there are ultrarunners.
I've worked with coaches in the past and all of them (3 different coaches) advocated lifting to some extent, usually with light to moderate weights and high reps.
A year ago, while surfing, I stumbled across Alwyn Cosgrove's blog. This led me to two other sites that I've grown to love. One of them is Precision Nutrition and the other is Lou Schuler's blog.
I discovered that Lou and Alwyn had written a book called The New Rules of Lifting. As I read more about the book, I realized that it was written for people interested in getting stronger, but the focus was on free weights - higher weights than I'd ever lifted - and on reasonable length workouts. As a runner, a book that promised I could be in and out of the gym in less than an hour, with a good strength workout behind me seemed very appealing.
I bought the book and really enjoyed it. Lou's writing style is easy to read and very entertaining. The book contains a variety of programs and defines progressions through programs. I've been following a progression for neophytes so far, and I've worked through 3 full training cycles and 2/3 of a fourth cycle.
In the past, I've always used machines (Universal or Nautilus) for the majority of my lifting. I would usually start lifting in the winters and after my peak mileage months hit, I'd quit. The primary reason I would quit wasn't that I couldn't fit the work in, but rather that I wouldn't see much progress. It seemed that I just didn't get stronger, so why put in the time?
I joined a local gym. I learned how to use a squat rack. I learned the value of chalk when doing deadlifts to prevent my palms from getting ripped to shreds.
And, I started to get stronger. I'm still not strong by any means, but I've made incredible progress in the gym this year. During the spring and summer, I lifted 2x per week. This fall, it's been 3x. Starting the first of the year, I'll probably cut back to 2x per week.
I have found that I've fallen in love with lifting. I'm stronger than I've ever been in my life. Moving real iron around in a way that uses lots of muscles is fun. I've learned to love the soreness in my glutes after a tough round of squats.
Does it help my running? Well, since I started lifting this year, I haven't lost any weight. But, if I am using calipers correctly, I've added about 8 pounds of muscle this year and dropped about the same amount of fat. I feel stronger when I run, even when I'm tired. My speed has improved as well.
This past summer, I ran my fastest ever 100 miler, a few months after I started lifting. I did this off a lower mileage training base than most of my other 100 mile finishes. Admittedly, it was on an easier course (Vermont) than some I've run, and I know that course well, but a PR is a PR.
I don't know if I ran that PR because of, in spite of, or with no connection at all to the lifting.
But, so far, I'm pleased with the results from my time in the gym and I'm going to stick with it.