You raise an interesting point. I have read just about every training book there is and there seems to be sharp disagreement among highly respected coaches about the pace of the long run.
Many such as Lydiard, Benson and Liquori say that the long run should be done at a strong aerobic effort (definitely not "slow" and not jogging), but at a slower pace than typical AT/tempo runs. They say the most important thing is time spent running on the long run which increases muscle capillarization and development of tiny beds of capillaries, increase the number of mitochondria in the cells and the ability to store glycogen. They say shorter tempo runs of 7-15 miles are the time for the hardest pace to increase heart stroke volume, heart size, and capillary density.
Many others such as Wetmore and Joe Vigil believe that the long run should not only be the longest, but the fastest paced or nearly fastest pace run of your weekly base building routine.
Who is correct? I don't know. Maybe it is not that significant. Maybe the real key is to run as many total miles per week averaging as close as possible to the anaerobic threshold as possible without breaking down and the details are not as not as significant? Very interesting question you ask that doesn't get talked about much.
By the way, I could care less about what most of the top of the heap Pro runners of today say they are doing concerning this issue. Many of the Pro runners are secretive and deliberately misleading about how they train. Also, drugs distort everything including what is the best way to train. You have to be careful, what is the best routine for someone taking drugs, will not be the best routine for someone not on drugs. For example, the best routine for a weight lifter on steroids is 25 sets per body part. The best for someone training natural is about 5 sets per body part. Another example is a natural runner during the anaerobic development phase who can barely handle one hard interval session per week but on steroids could handle and should ideally do 3 hard intervals sessions per week. Another example is Lagat running 3:44eq mile off of 40 miles per week. I can absolutely guarantee that 99% of you will fail to reach your full potential if you follow Lagat's 40 mile per week routine. The point I am making is to be wary of trying to copy the program of one of today's Pro runners. You are better off reading Lydiard's books, Liquori's book, Benson's books, Once a Runner (based on Shorter's and Jack B's training), Running with the Buff's (Outlining Wetmore's training) and Joe Vigil's book. Then go out and experiment and see what work's best for you.