If there's one thing that I don't like to do in running, it would be training myself to run fast. In order to do that, you need to... run fast. I'm a pretty laid back kind of guy, and I don't like to work too hard too fast. Even if it is only short distances. I like going nice and easy, and if I were given the choice to run 20 miles slowly, or 5 miles as hard as I could, I would definitely take the 20 miles. However, there's a need for me to build up my speed. I need to be able to become fast, so that when I go for the 12 hour race, I would be used to running at such a fast pace, that when I go slowly, I would be able to last longer. There are three paces that I look at when I run. Your Max VO2 Pace, your Lactate Threshold Pace, and your Running Economy Pace.
Your VO2 Pace is the pace at where your muscles are using the maximum amount of oxygen. The higher your pace is, the faster you will be at running. Your Lactate Threshold Pace can be anywhere between 70-80% of your VO2 pace (according to some research... but it varies depending on the sources), depending on what level of a runner you are, and it determines how well you race. The threshold is the pace at which the accumulation of lactic acid isn't fast, but slow and steady. Finally, the Running Economy Pace. It is a pace at which you are running about 65% of your max VO2, running fast enough to train your muscles to run, but slow enough that it doesn't work on your threshold.
Basically, the VO2 pace is the fastest I can run using my muscles efficiently, but at the same time, wearing them down fast. If I raise my VO2 pace, then I will be able to vicariously improve my lactate threshold pace, and my running economy pace would also go up. If I spend some time on the VO2 once in a while, I would be able to improve my running overall. Just for fun, I looked at the University of Tennessee Track Club records, and I figured that if I could go for a good time on the 10,000m record, it would help my overall running. Although there is no way I can bring my time down in a month, I believe that in a year, if I could get to that point, then I would be able to do a lot better with my ultra-marathons. When you're reaching to better yourself, you're going to have to do things you don't want to do. You're going to have to attack it over and over again, owning it, and making it yours, so that you would be able to conquer that and use it to make you into a better person.
Today, I spend about a half an hour doing interval training, running hard one minute, and taking a break for a minute, and doing that 10 times, with a 5 minute jog before and after... it was tough... but it was worth every run.