From Competitor Running (link):

"On paper, a session of 8 x 400-meter repeats gives the illusion of an easy workout; but executed properly, these one-lappers can be a real doozy.

The workout, made popular by former marathon world-record holder Rob De Castella of Australia, is as simple as it sounds: eight, one-lap repeats of the track. The pace: it varies (more on this in a bit). The recovery: a scant, but swift, 200-meter float.

So how fast should you run your repeats? De Castella recommends anaerobic threshold pace, or your fastest maintainable speed. For most runners, this roughly translates to a speed that is slightly faster than your current 5K race pace. Doesn’t sound too difficult, right?

Here’s the catch. The recovery between each repeat is a 200-meter, or half a lap, “float” – not a walk or slow jog, but more of a brisk trot. The key to this session is that you never completely recover before starting the next 400-meter interval, and as the workout progresses you’ll be forced to work harder to maintain the same pace at which you started out. This is threshold training at its truest, simulates race surging and will teach your body to recover quickly while running fast.

“As you get fitter, your red line rises from about 80 percent of maximum heartrate to 90-95 percent,” writes world-renowned running coach and top exercise physiologist, Jack Daniels. “Physiologically, threshold training teaches muscle cells to use more oxygen–less lactate is produced. Your body also becomes better at clearing lactate: race-day red line speed rises.”

Beginner

* Warmup: Run easily for 10-15 minutes; follow with 4 x 20-second strides.

* Workout: 4-6 x 400 meters (one lap of the track) with a 200-meter “float” between intervals. Run the 400’s at 1-2 seconds per lap (4-8 seconds per mile) faster than your current 5K race pace and the 200’s at a pace that’s about a minute per mile slower than your 5K pace.

* Cooldown: Run easily for 10-15 minutes, stretch, refuel.

Advanced

* Warmup: Run easily for 20-25 minutes; follow with 6 x 20-second strides.

* Workout: 8 x 400 meters (one lap of the track) with a 200-meter “float” between intervals. Run the 400’s at 1-2 seconds per lap (4-8 seconds per mile) faster than your current 5K race pace and the 200’s at a pace that’s about a minute per mile slower than your 5K pace.

* Cooldown: Run easily for 20-25 minutes, stretch, refuel.

This workout benefits a wide range of runners from weekend warriors hoping to improve their 5K time to serious marathoners looking to lop minutes off their personal best. De Castella used to perform this workout weekly, but for most runners, tackling such a session every other week or even once every third or fourth week is plenty. As you get fitter, your lap times—and your race times—will get faster."

I love 400s.  They make me feel fast and I don't die from asthma while doing them.  Contrary to 800s.....

I do 3 miles warmups, for obvious reasons.  I don't think I have done more than 8 sets, although when I used Daniels' 10K program I think I had 12 sets but couldn't complete them due to asthma that day.  I want to give them a shot again and see.

Perhaps the article mentions De Castella to illustrate that marathoners can also benefit from this workout, but it or variations on it date back to the 50s. Mihaly Igloi popularized it when his runners dominated U.S middle distance and were a huge force on the world stage. It lost its following in the 60s and 70s when distance runners were attracted to Lydiard's methods. My guess is that most people reading the article, if they've even heard of De Casella, had no idea that he ran that type of workout.

As far as the terminology, there is more than one threshold and the article is clear enough that LTH apparently understands it well enough that he thinks it's a good workout.

Words often have different meanings for the speaker and the listener.  But the workout is clear

Workout: 8 x 400 meters (one lap of the track) with a 200-meter “float” between intervals. Run the 400’s at 1-2 seconds per lap (4-8 seconds per mile) faster than your current 5K race pace and the 200’s at a pace that’s about a minute per mile slower than your 5K pace.

Why bother debating the preamble?

I love 400's and did many over this past summer.  Intervals have built in variables - distance run, the pace of that distance, rest interval, the pace of that interval, and the total times repeated. I ran my 400's faster and had a "slower" recovery interval.

I did my 400's at mile pace which is in line with McMillian. I would say my workout was more of a VO2 workout.

This article almost makes it seem like a threshold workout because even though the run distance is at a slower pace, the rest interval  is "floated" at a faster pace.

I would think the way I ran them would have a greater range in heart rate whereas this workout would keep the heart rate in a tighter band - which to me would be a threshold type run.

These are just my thoughts. Hopefully a lot can be learned from this discussion as 400's are a very good workout!

The Daniels reference does seem to be taken almost out of context.

I like the 600s better than the 400s since you have a longer sustained period at VO2max

These are on my agenda for tonight.