Here is a pretty quick read from Competitor Running on why you should consider training across all paces if you currently do not.  Pay specific attention to the parts on the recruitment ladder and fatigue.

Most of you know this already, but for those who do not maybe it can help you:

Types of Muscle fibers

Skeletal muscle, the type that is responsible for moving our muscles when we run, is comprised of three different muscle fiber types, each with its own advantages, disadvantages and specialty.

Type I, better known as slow-twitch fibers, are the body’s primary method for less explosive, sustained movements. They do not contract forcefully and thus require less energy to fire, which makes them well suited to long distance running. More importantly, they house our main supply of oxygen-boosting power plants — mitochondria, myoglobin and capillaries.

Type IIx are best known as fast-twitch muscle fibers. These are the muscle fibers primarily responsible for fast, explosive movements like sprinting. However, they lack the endurance-boosting ability of slow-twitch fibers and can only be used for short periods of time.

Type IIa are what we call intermediate fibers. These are a blend between fast- and slow-twitch fibers. They have some aerobic capability, but not as much as the slow-twitch fibers, and they can fire more forcefully, but not quite as explosively as the fast-twitch fibers.

Each individual has a genetic predisposition to certain muscle fiber types. The misconception that many runners have is that each fiber type is exclusive (i.e. you can only use one or another), we can’t train to improve how our fibers function, or the alter percentage we have of each. In reality, with the right training, we can manipulate and improve all three.

How Muscle Fibers Work

Before we can outline how to improve fiber function and conversion, we must understand under what circumstances we use each fiber type, when they are recruited, and when they change.

Recruitment Ladder

This process starts with what we call the recruitment ladder. The recruitment ladder is a way of envisioning how and when each fiber type is activated. At the bottom of the ladder, we have the slowest, least explosive fiber type, Type I slow-twitch, and at the top you have fast-twitch fibers.

You move up the ladder based on how much force you need to generate to sustain a given pace. If you were to head out to the streets right now and begin running easy, your body would start by using slow-twitch fibers. If you were to pick up the pace, your body would start recruiting some of the Type IIa intermediate fibers to supplement the need for more power from the muscles to generate more force during the stride. Finally, if you were to sprint across the road to beat traffic, your body would then engage the fast twitch muscle fibers to give you the explosive burst you need to sprint.

Fatigue

In addition to intensity, the other factor in muscle fiber recruitment is fatigue.  As you get further into a long run, the slow twitch fibers you’ve been using start to get tired and you can no longer fire them as efficiently. As a consequence, you start to recruit some intermediate fibers to help maintain pace. Of course, these intermediate fibers require more glycogen and are not as fatigue resistant as slow-twitch, so it won’t be long before you find yourself slowing dramatically as your muscles start to fail.

Now that we better understand the different muscle fiber types and under what circumstances we use them, we can employ this knowledge to better structure our training. In this section, we’ll outline some of the more common workouts, identify which muscle fiber they target, and what this means for your running-specific fitness.

Long Runs

The long run targets the slow-twitch fibers, making them more efficient by building their aerobic capabilities, and also making them more fatigue-resistant. Continuous long runs also help convert a greater percentage of your muscle fibers into slow-twitch fibers, which is one reason you continue to get better with years of mileage.

Tempo Runs

Tempo runs target slow-twitch and intermediate muscle fibers. Slow-twitch fibers reach maximum recruitment and contraction speed at tempo pace, which is one reason why tempo runs are so critical to endurance training. In addition, tempo runs help improve the recruitment patterns of intermediate fibers with slow twitch fibers. In essence, it improves the ability of both fiber types to work together for maximum effectiveness.

Short Repeats

Traditional interval workouts like 12 x 400 meters help recruit intermediate and fast twitch muscle fibers. By being used together, these two fiber types learn to interact more efficiently by reducing activation of unnecessary fibers. More importantly, it improves our neuromuscular coordination — the speed at which the brain can send signals to the muscles to fire, thus making you more efficient.

Speed development work, like strides, hill sprints, and short, maximum effort sprints on the track, help recruit the maximum amount of fast-twitch muscle fibers. While this may not seem like a necessary benefit to someone running the marathon, this type of training makes each stride more explosive and enables you to generate more power without increasing effort. This increased power is what makes your stride more fluid and efficient.

Drills, strength training and dynamic stretching improve recruitment patterns (the ability of your muscle fiber types to work together concurrently), increase strength, and reduce inhibitions (discussed more in-depth in this article).

By incorporating this type of work in your training, you develop more biomechanically sound running mechanics, become more efficient, and train your fiber types to work together.

Runners typically talk about training in terms of metabolic improvements — aerobic development, VO2 max, and lactate threshold — but we often forget the important role muscle fibers play in our fitness and ability to run faster.

The next time you plan your training, don’t forget to factor in how you can better train your muscle fibers as well.

Link to full article here.

I have always asserted that every distance runner can benefit from doing speed work even if they don't care a bit about getting faster.  I think it helps to prevent injuries and shaking things up a bit makes running fun.

Well this kind of stuff tends to make my eyes glaze over. I keep telling myself I need to learn more about it, but just can't focus.

shaking things up a bit makes running fun.

But, this. I am on a formal training plan for the first time, and have found myself really looking forward to speed day. A 10 mile easy run can really feel like a chore, but an interval session totaling 10 miles (with wu/cd) feels much more inspired and goes by more quickly (well of course time-wise it actually does). Every repeat is like a little race against your target split. For the longest time I was sort of intimidated by intervals, I don't know why, but now I know what I've been missing all this time.

Well this kind of stuff tends to make my eyes glaze over. I keep telling myself I need to learn more about it, but just can't focus.

Which is why this particular article caught my eye, it is written to the point and is easy to digest.

As Z stated this same information is in the Hansons book, but with my ADD, non-paying attention, easily zoned out ass I never absorbed it because of the way it is written there.  While the two paragraphs I mentioned in the original post are written "in English" so to speak.

I have long since embraced the concept of training across all paces but was never bothered with the why, I just knew that I should.  Here, the why is explained in a manner where even a cave-man can grasp the concept.

I don't really understand this, but I dedicate weeks or months to all distances from 5k to the marathon, so I think I have all my bases covered.

This also points out a flaw in Daniels' approach and his concept of "quality" junk miles if you aren't training within his specified ranges.

 

An aside to LTH: Sorry but Daniels is not infallible.

I've always wondered about this. If you run too fast or too slow for a particular workout you may not get the intended benefit or work the system you were supposed to for that particular day, but in the grand scheme of things you are still doing something and that will help in the long haul.

I have never argued that Daniels was infallible but I absolutely agree with his assertion that, before you begin any run, you think, "what is the purpose of this run?"  His VDOT tables give a single pace rather than a pace range but I don't know of anyone who doesn't realize you're going to have to adapt depending on the day.  No one will run as fast on a hot day or a windy day as they would on a cool, calm day.  But, here's what he's talking about regarding pace.

 

Let's say your "I" pace is a 6:00 pace but today is a perfect weather day and your legs feel great so you end up doing most of your repeats in the 5:50 range.  What did that accomplish?  Well, let's start with what it didn't accomplish.

 

The purpose of and "I" workout is to improve your VO2max.  Note that there is a velocity at which you hit VO2max.  You can run faster than that.  Indeed, world class runners will run a 5K a bit faster than their vVO2max.  Given that your vVO2max is the pace at which you could race for about 15 minutes, almost everyone would race a mile faster than their vVO2max.  But, even if you run faster than your vVO2max, you don't do anything extra to improve your VO2max.  100% is 100% and you can't use more than that - a coach's admonition to "give it all you've got and then some" notwithstanding.  So, if you had run those repeats at 6:00, you would have had the same physiological impact in terms of VO2max development as you did with the 5:50 pace.  You gained nothing by running faster.

 

What you also did not accomplish is to gain the benefits of an "R" workout - what some call "speed intervals" or "speed repetitions."  Those are done at roughly mile race pace and if your "I" pace is 6:00, your "R" pace will be around 5:30 so 5:50 is way too slow to gain the benefits you seek from an "R" workout.  (For those who haven't read Daniels, an "R" workout is actually a good bit easier than an "I" workout because the intervals only last anywhere from 45 seconds to 2:30 and you take full distance recoveries where as "I" workout intervals last 3-5 minutes and you take perhaps half distance recoveries).

 

Of course, because you ran each interval at a pace that was 10 seconds faster than prescribed, you did place additional stress on your body.  So, let's add this up.

 

Additional VO2max improvment?  No.

Benefits of an "R" workout?  No.

Additional stress on your body?  Yes.

 

In other words, you got exactly the same benefit you would have gotten had you run at a 6:00 pace but you placed additional stress on your body and additional stress = additional risk of injury.

 

I would agree that a valid critique of Daniels, at least in the 2nd edition (I haven't gotten the 3rd edition yet) is that he doesn't prescribe any workouts across various zones.  Many coaches will have workouts that mix up tempo running with VO2max running and even speed repetitions and Daniels doesn't have any of those types of workouts.

I'm well aware of Daniels' arguments. My contention is his avoidance of training in those zones between I and R pace, between R and T pace. Workouts need not be so targeted as Daniels prescribes and in fact studies such as this one suggest there is a great deal to be gained training across all paces.

Thanks the info LRB

Let's look at the Daniels gaps plugging in my 5K PR.

E - 8:01

M - 6:49

T - 6:26

I - 5:55

R - 5:30

The biggest gaps I notice are between E and M and between T and I.  I have run plenty of runs too far in that gap between E and M when I am feeling particularly good.  I'm talking about running 5-6 miles around a 7:25 pace.  If I don't have anything scheduled for the next day other than an E day, no big deal.  But, if I do have something scheduled for the next day, that's enough to knock me out of being able to do a hard workout.  I simply can't run 5 or 6 miles at a sub 7:30 pace and then do a hard workout the next day no matter how great I felt when I started that run.  I honestly think there is very little to be gained by running at a pace that is significantly slower than your marathon pace yet so much faster than your easy pace that it negatively impacts your ability to do a hard workout the day after.

As for the gap between T and I pace, I find that more problematic - especially if you are training for a 10K.  The T pace is not much different from HM pace so if that's what you're training for, you'll get a lot of race specific training.  The I pace is not much different from 5K pace so same thing there.  And the M pace, well duh.  But, there is nothing specifically for the 10K.  A 10K is a good bit faster than a HM but still a good bit slower than a 5K yet Daniels has no runs in that pace range.  Moreover, in terms of building what I would call mental resistance to fatigue, I think you'd be well served by doing some 10K specific workouts like McMillan suggests here.