From the Coach guide
The longest run of the week, at an easy effort.
Long runs build endurance. Your body learns to keep going as you get tired. Research calls this durability.
Run at your easy pace. Start slower than you think you need to. Take water and food on very long runs.
Most plans have one each week. It grows the most across a block, inside Coach's limits on single runs.
For a half or a marathon, every other build or peak week has a race-pace long run. Run the first three quarters easy. Run the last quarter at your race pace. It is that week's hard run, so the week has no other. Coach chooses this pattern to practice race pace while leaving recovery time.
Heart rate drift: Coach measures how much your heart rate rises during a long run. That reading is in Heart rate drift.
Review
The authors call the ability to hold your fitness as you get tired durability. They argue that tests on rested athletes miss it.
The review proposes the idea and how to measure it. It does not set how long a long run should be.
Coach