Roy Wallack, author of "Run for Life," has a very persuasive section on the use of ultra-intervals -- intervals in which you run all-out for 20 to 30 seconds, followed by a minute or so of light jogging or walking. You're supposed to do eight of these, and it's supposed to have a magical effect on things like your VO2 max and muscle power.
Intervals are nothing new, as Wallack concedes; the difference here is that you are REALLY supposed to push yourself, to the point of feeling like you're going to pass out.
This is my third or fourth ultra-interval workout. I must say, it's pretty hard to get to that almost-passed-out feeling when running on Seattle's sidewalks. Going that fast down the sidewalk on a summer morning is courting disaster -- you'll surely crash into somebody on their way to work or get tangled up with a dog-walker. And although I programmed my watch for 20-second intervals followed by 90-second recoveries, to actually follow this would have also been dangerous -- I hit some of those 20-second sprints right as I reached an intersection. So instead I tried to sprint a block, walk a block. I'm pretty sure I never hit the all-out, gasping-for-breath moment. A workout like this might best be done on a track, or perhaps around Green Lake.