As with a lot of training approaches, runners have a very polarized approach to how they handle long runs.
Either they do a simple easy run over a long distance, or they add in some tempo with the long run (either trying to run the whole thing at a tougher moderate pace, or mixing in tempo segments with easy running), turning it into a grueling exercise.
Both polarized approaches have substantial drawbacks.
The long easy paced run may develop long aerobic endurance, but it also accustoms you to only handling your longest distances at an easy pace. Any attempt to race longer distances thus becomes a huge struggle, because you haven’t practiced running faster at max distance.
The mixed tempo run may address that issue, but creates another issue: It asks you to work especially hard at points on a run that is already fairly difficult due to its duration. This increases the burnout and injury risks, and at the least makes long runs such a miserable experience that many just forego any sort of intermediate tempo work on those runs. (It’s the biggest issue with the Daniels Marathon Plans. Those quality long runs are super-demanding. Few outside of elites and hardened distance running vets can consistently handle them.)
Regular readers can probably sense where I’m going with this point: There is a vast and mostly-unexplored middle ground to long runs that will allow you to work on and develop aerobic strength (aka the ability to maintain faster paces over longer distances), without demanding so much from you.