Threshold training runs are commonly described as “comfortably hard” (82%-88%) and is completed at a speed that you can race for about an hour. Why is this an important part of distance training? Because this helps build your endurance. If you stress this area of training you can increase the speed that you can race for an hour; or race at a faster pace more comfortably. This workout can be a challenging for many athletes as its “comfortably hard” and it can challenge you mentally to stay on top of the pace.
I visualize it as the half way point to the critical point (75%) of a race; that is usually where many start to drop off the pace. You should focus on belly breathing, proper posture and form during this physical and mental challenge.
Dr. Daniels explains it in detail in this video: