The questions every runner should ask themselves is "Why am I doing this run? What is it's purpose? What will I gain from it?" If you can't give an answer then you need to learn why. As with many things in life, knowing why makes you more invested in making sure you carry out the task in an ideal fashion.
I've made a few posts on similar topics before that cover the general ideas:
Eureka! The Quintessential Running Post
Train slow to race fast: Why running more slowly and capping the long run at 2.5 hours may dramatically improve your performance
The Marathon is 99% Aerobic (and 95% for HM and so on): So how to train for it!
And my favorite resource (runners connect blog):
How Fast Should Your Easy Runs Be?
Why Running Harder Won’t Help You Get Faster
Are You Sabotaging Your Long Run by Running the Wrong Pace?
How to Know if You are Running Easy Enough on Your Recovery Days
How Running 80% Easy Could Make You 23% Faster
Jeff Galloway- Go Slow To Go Fast (podcast)
3 Simple Ways to Determine if You are Running Easy Enough: Matt Fitzgerald (podcast)
In it's most basic form, a running training plan is meant to provide a "stimulus" to the body and mind. The "stimulus" depends on the type of training you do. You do speed training and you elicit the potential for changing your muscle fibers from speed focused to endurance focused (can take years to make this transition). You do lactate threshold pace training and your ability to run a certain pace for about an hour becomes faster. You do a run in super hot conditions over many days and you're body thins out your blood so the blood can travel throughout the body easier and the heart doesn't have to work as hard. Running multiple days in a row causes your body to be in a state of mild fatigue which over time puts you at a different "starting position" for different runs (i.e. instead of starting at mile 1 for your long run, you're actually starting at mile 8, OR said another way if the fatigue build-up in your body is zero going into your long run you might never teach your body what it's like to run beyond your long run mileage **However, with that being said I still place a max of 2.5/4.5 hrs on training runs dependent on style**. It's why the Hansons plan can go to 16 miles max (for some people) and still enable you to be capable of completing a 26.2 mile race.
All of these stimuli lead to an adaptation in the body in response to that stimulus (not everything is listed here as adaptations, but a short example)
Speed -> Fiber change
Lactate Threshold -> Fatigue Clearance and tolerance of fatigue
Hot/Humid Conditions -> Thinned out blood
Easy runs -> Increased capillaries
Long Runs -> Mitochondrial density and volume
Glycogen Depletion Training -> Use fat more readily as a fuel source
So, now think of everyone's pace spectrum. The universal truth is everyone has a VO2max. Everyone also has % VO2max that can be attributed to different race distances. When you train at different paces along the %VO2max spectrum it elicits different responses (like stated above) based on how you accomplish a workout. So to be a well-rounded runner you want to touch bases on lot of different paces across the spectrum. This will allow you to elicit lots of different benefits on different days. As the race gets closer, you specialize. The Dopey can be viewed as a special event because it has a 5k, 10k, HM, and M. But at the end of the day, training for a marathon will sufficiently train you for the other distances. This is because at heart the marathon is 99% aerobic and even the 5k is considered ~80% aerobic. So doing lots of aerobic work will yield benefits for both distances. But training more at 5k pace means you're unlikely to garner the same benefits necessary for the marathon. The cutoff for primarily aerobic work is around M Tempo to long run pace.
The use of the run/walk method does not change having a %VO2max association with race distances. I liken the use of run/walk to speed intervals. I do personally struggle with trying to make the complete connection between the two though. Is someone's 5k run/walk average pace their associated %VO2max 5k pace? If so, then by virtue of the walk, it means they have to run faster than 5k pace to complete a 5k at 5k average pace. This means the run portion must be something like 1 mile or 3k pace. This changes the game in how the body adapts to the training. Take for instance Galloway's long run recommendation of M Tempo + 2 min. Why 2 minutes? A lot of other people tend to recommend 45-60 seconds not 120 seconds. It is my guess that it can be traced back to the use of run/walk. I don't concern myself with the average pace but more so how did that average pace get accomplished. So a person who is doing a M Tempo + 120 seconds must be forcing themselves to either -
A) Walk slower
B) Walk for a longer duration
C) Run slower
D) Run for a shorter duration
Since Galloway allows self-selection of pacing, then I believe most people will gravitate towards either running slower or decreasing the duration of the run (*well and because Galloway's instructions say use the same run/walk between maintenance runs and long run means the only options really are A and C). By doing this, it means that the run portion will likely shift from being in the 10k/HM tempo physiological %VO2max zone into the M Tempo/long run physiological %VO2max zone. He accomplishes the same task others are asked to do by pushing his long run to a much slower scheduled pace (45 vs 120 seconds + M Tempo). The same can be said for his maintenance runs as
@Dis5150 has described them. He has you do some M Tempo pacing using run/walk which is really intervals of ~5k/10k physiological pacing. But 5k pacing (i.e. run at mile/3k pacing) on those maintenance runs would be much too fast and not elicit the benefits that would be most useful on race day.
The other thing I found interesting is his more recent recommendation that the walk should be capped at 30 seconds (it's in the podcast above). According to his data (which is not publicly available), people tend to fade less when the walk is 30 seconds. Why is that? My guess is because people find that shortening the walk to 30 seconds forces their run to either be shorter or slower to be able to still accomplish the distance within his described "effort" guidelines (it's also one reason he's a big proponent for group running on the long run because you're suppose to be able to carry on a conversation at his M Tempo + 2 min). I've worked with a few people on here who are capable of incredible workouts of 60 sec run at what should be their mile pace and 30 sec walk. I know personally that workout would CRUSH me. If I run at mile pace, I usually need 2.5-3 minutes of walking recovery before I can even consider another one. So I applaud those than can do it. I think (and again just my opinion) is that Galloway has found that shortening that walk makes people run slower during the run, but it improves the final average race pace because there is less of a fade.