COMFORTABLY HARDOne of the important components of an overall training strategy for a runner is the "
tempo run". The tempo run is basically the opposite of the "long slow run". The tempo run is designed to challenge your metabolic system to become more efficient at using oxygen, by taking you to the threshold of exhaustion but not going over that threshold. There is a minimum amount of sustained time which is required for the tempo workout to be effective -- about 20 minutes.
The standard tempo workout consists of a 15 minute warmup, a 20 minute tempo-pace run (at about an 8 out of 10 on the effort scale), followed by a 15 minute cooldown. The "8 out of 10" is also characterized as "comfortably hard", meaning you are pushing your pace a bit, but not quite going all out. Basically, you need to run as hard as you can reasonably sustain for 20 minutes without being totally wiped out -- maybe about 80% of the effort you would give if you were running a 5k race.
As your fitness improves, you gradually increase the 20 minute tempo period. In a full-on marathon training cycle, ideally you should be hammering out a 10-mile stretch of tempo effort at least twice during a long training run. I'm not quite there.
I've never focused much on tempo training, because I've been so concerned about my endurance per se. I've been way more concerned about being able to stay on my feet for 3 or 4 hours (or more), and never worried too much about my speed. In fact, during the explicit "tempo" workouts I've done, I've chased people who are too fast for me and I've become extremely frustrated. Recently, however, I've had some good tempo experiences and I'm convinced I need to focus more on this aspect of my training!
For my tempo runs, I usually just run around Greenlake (with a warmup and cooldown of course). The inner path of Greenlake is about 2.8 miles. It takes me just a little more than 20 minutes to run this circuit. By running the same distance every time I do a tempo workout, I can guage my improvement. If my overall time is lower than the last (assuming the same level of perceived effort), then I may conclude that my lactate threshold has improved.
Here is a sampling of a few tempo runs I've done in the past few months.
Back in July of 2008, just a few weeks after my disastrous
first marathon, I tried to get back in the saddle and train seriously for a fall or winter marathon. It looks like I had a fair amount of residual fitness, because my splits were relatively fast for me (avg = 7:54), but it also looks like I ran out of steam:

After a summer of complacency, however, I slowed down. By the middle of September I couldn't even keep a 3-mile tempo run under 8 minutes per mile (though at least in this instance my splits were pretty even):

A week later I improved a bit, and was especially happy with my even first two splits, but disappointed that I apparently had more in the tank because I went way faster in the third mile; I should be more even:

I basically repeated this same pattern three months later -- just a couple weeks ago. I ran the first 2 miles at about an 8 minute pace, then bumped it down to 7:32. Weird. But I felt good!

And then finally this week I felt like I really did the tempo workout properly. My splits were more even, and more importantly I felt like I really ran the entire circuit at a "comfortably hard" pace. I was breathing hard, but could still mutter short sentences:

For my next few tempo runs, I'm going to set 7:20 as my baseline pace and just try to sustain that. Gradually, I'll turn it into a 25 minute workout, then a 30 minute workout, and beyond.