I'm going to start with addressing your edit first re: some scientific studies. You're right research on this type of stuff often leaves something to be desired. However, long distance running is actually one sport we have a lot more research than most.
Notice the theme of these studies, bolding mine:
"Like Slovic, other investigators have demonstrated a relationship
between quantity of training mileage and final time."
- PHYSICAL TRAINING AND PERSONALITY FACTORS AS PREDICTORS OF MARATHON TIME AND TRAINING INJURY
"The better runners were seen, on the whole, to have been running
longer, and to have more strenuous regimes, both in terms of intensity
of training and distance run per week."
- Anthropometric and training characteristics of female marathon runners as determinants of distance running performance
"Marathon Performance Time was also negatively related to previous
marathons completed (r= -0.47), workout days (r= -0.47), two a day
workouts (r= -0.52), total workouts (r= -0.56), km per workout (r=
-0.58), total training min (r= -0.56), training pace (r= -0.66), max km per week (r= -0.70), km per week (r= -0.74), km previous 12 weeks
(r=-0.74), and km per day (r=-0.77).
Marathon Performance Time for our population of runners may be
predicted (r= 0.82, R2= 0.68) by the following equation:
MPT, (min) = 449.88 - 7.61 (km per day) - 0.63 (training pace)"
- MARATHON PERFORMANCE IN RELATION TO MAXIMAL AEROBIC POWER AND
TRAINING INDICES IN FEMALE DISTANCE RUNNERS
"Marathon performance correlated to 5-km, 10-km, and half-marathon
performance and to years training, average and peak weekly distance,
number of weekly runs, and number of runs ≥32 km for women."
- Training Characteristics of Qualifiers for the U.S. Olympic Marathon
"the average weekly training volume in kilometers (r2 = 0.224, p <
0.01) and the personal best time in a marathon (r2 = 0.334, p < 0.01) were significantly associated with total race time, whereas no
anthropometric variable was related to race performance (p > 0.05)"
- Training Volume and Personal Best Time in Marathon, Not
Anthropometric Parameters, are Associated with Performance in Male
"body mass index (r = 0.24), the sum of eight skinfolds (r = 0.55),
percent body fat (r = 0.57), weekly running hours (r = −0.29), weekly
running kilometres (r = −0.49), running speed during training
(r = −0.50), and personal best time in a marathon (r = 0.72) were
associated with race time. Results of the multiple regression analysis
revealed an independent and negative association of weekly running
kilometres and average speed in training with race time."
- What is associated with race performance in male 100-km
ultra-marathoners – anthropometry, training or marathon best time?
"In men, the mean weekly running distance, minimum distance run per
week, maximum distance run per week, mean weekly hours of running,
number of running training sessions per week, and mean speed of the
training sessions were significantly and negatively related to total
- Sex Differences in Association of Race Performance, Skin-Fold
Thicknesses, and Training Variables for Recreational Half-Marathon
So one reason to engage in long, slow, distance training, is because it helps you run more miles, and the more miles people run, the faster their times tend to be. However, that doesn't mean it's optimal or the cause of faster times.
One reason to not immediately buy into this philosophy is it's really quite different than other human performance events. There are far and few events where merely doing the event more and more will keep increasing performance.
I'll give two areas for food for thought.
- For non-professional athletes, time is a limiting factor.
It's not easy fitting e.g. four+ hour workouts into a week.
You almost always find everyday people basically do the best they can during the work week, then on the weekend have one long workout, trying to simulate the event the best they can.
This is why first timers to these types of activities often end up destroyed after the event. By far the biggest mistake I see first timers do is,
"Eh, I mean, I've done a 10 mile run in training. I think I'm ready
for a marathon."
This is no different than a lifter going,
"Eh, I mean, I've squatted 200 pounds in training. I'm sure I'm ready
for 400 pounds. Or more."
You're talking more than a 100% increase in volume, overnight. Cue the stress fractures.
In other words, the average rec person has a hard time getting the required volume in, then ends up delusional thinking what they've been doing is enough. Until they get hit by a wrecking ball on event day. The more running they can do before the event, the less likely this happens, the better they do.
- Body weight
Again, more a concern for rec people. The more they run, the more likely they are to weigh less, the better performance in longer distance events tends to be.
That is, a person could likely do just as well running less with more attention to their eating.
In fact, there is really only so many calories a person can burn from exercise. (See: New insights as to why you can't avoid exercising.) The idea someone burns e.g. 10,000 calories is false. Where I wouldn't be surprised if those engaging in more training simply do two things 1) blunt their appetite more 2) have less time to eat.
One way to back this up is in many of the above studies you can see anthropometry is not correlated with performance. Well, at the elite level, we know that's not true. Kenyans dominate, and they have a very specific body type.
For the majority of people engaging in long distance competitions, the ability to merely do the event is an enormous hurdle. The ability to then run the whole thing is another hurdle. The ability to do that with any level of regularity and not got hurt yet another. Finally, we have speed.
You will see average running pace is also correlated with performance in the studies above. Speed of course matters. But for the average person looking to run these distances, it's not a priority. Finishing without having to limp for a month afterwards is.
Say you're an everyday person following a common marathon training plan, where there's one long run on the weekend. Your options are:
- Run 10+ miles at a relaxed pace
- Run 10+ miles at race pace or faster
The second involves more injury risk, and a greater chance you won't even hit the distance. Marathon training has an absurd injury rate. You have to get people to the distance before you worry about speed.
But once someone's body can regularly handle the distance, then you're right. Speed needs to be addressed, and not just in shorter workouts. (That said, even at the elite level, they will often run longer distances slowly, just to maintain the ability of their body to run that far, while lessening the stress of how intensely they run it. You can't run hard every workout.)