The article (and many threads on letsrun.com) convinced me that the long run is not the key workout. It's important, but not as important as the MP runs and the speed/strength intervals (intensity!) and far less important than total weekly mileage (volume!).
I've heard a couple of reasons to do 20 or 22 mile long runs:
1) 20 miles is where you hit the wall so running 20+ miles trains your body to not bonk or hit the wall (or variants of this). Indeed, 20 miles is about the distance that our muscles run out of glycogen but we avoid this in a race by taking gels during the race, not by running 20+ miles during training.
2) 20+ mile runs get us mentally prepared for the distance and allows our legs to adapt. But we're not doing the 20+ mile runs at race pace during training so how does a long, slow 20+ miler prepare us for anything other than long, slow runs? From all I've read and heard, it's the last 6 miles that make the race, not the first 20. Adding 10 miles of MP to our long run is a good training strategy but how close does these come to preparing us mentally and physically for racing those last 6 miles (and what is the toll on our bodies during the next week)?
I suppose if one's goal were to simply finish a marathon, a 20+ mile run may be useful mental preparation, but that's not my goal. Rather, it's to run it as fast as I can. And to run it as fast as I can, I need to have my legs prepared for the other weekly workouts. And if I'm tired and sore from a 22 mile long run, I cannot do these workouts as they should be done.
20+ mile runs do have their place on a marathon training schedule...if you run 80+ miles per week. A good rule of thumb is that the long run should be about 20% of your weekly training volume. Or 25% maximum. For our sub 2:30 marathoners running 120 miles a week, a 22 mile long run is nothing; these guys are running 20 miles most days of the week (but in doubles). As a percentage of weekly volume, it's equivalent to a 7.5 mile run for someone running 40 miles per week. But a 20 mile run for someone running 40 miles per week seems crazy. If you throw in a proper interval day and a MP day, you've pretty much exhausted your mileage total and you'd have to rest the other days. I don't think 3-4 days of running is going to get you your fastest marathon.
Many coaches combine a 6-10 mile MP run with the long run. I've followed the Hanson's schedule and separated these. I don't disagree with the combination, I've just not done it. So my long runs have all been easy pace and generally <=16 miles. I did one 18 miler just because. My weekly mileage is about 15% more than the Hanson's advanced schedule but those miles have been added to the other weekly runs and not the long run. Most of my long runs have been on trails. This has kept me out of traffic (like) and added much more hill work, which is good for maintaining strength. Plus its much easier to keep it at an easy pace on trails than on the road. I've run only two long runs on the road. Two of my long runs have been mountain runs (Mahoosuc and Pemi loops). Both of these runs included significant walking sections and both short (Mahoosuc) and long (Pemi) rest breaks. But both were significantly harder than a 16 miler on the road.
There are many, many reasons that I might limp in 5-30 minutes slower than my goal time, but not running a 20+ miler is not one of them.
P.S. a good question is why not do a 20+ mile long run if it is >> 20% of weekly volume. My decisions rested on two reasons:
1) recovery time. 20+ mile runs, especially if they are on the road and at a decent pace, are hard and recovery can be long. I didn't want my long runs to compromise my midweek workouts (intervals and MP runs)
2) risk of injury. Given my increased weekly mileage, which by itself increases my risk of injury, I didn't want to further increase this risk by running 20+ mile long runs.
We all vary in our ability to recover and resist injury. If you recover quickly from a 20+ mile run (or you don't care about midweek workouts) and if 20+ mile runs don't seem to increase your rate of injury, then a 20+ mile long run is a great tool to have in your running toolkit, regardless of your weekly mileage.