What's the difference between Z4 threshold pace and Z4 10K pace?

I also have the same doubt. Shouldn't Z4 be determined by your HR (80-90%) and not by the distance?

Hi, When there's a workout that has two sections at threshold pace, but one is at 10k pace, I treat the 10k pace section as a slightly harder effort. 

No hi ha contestació de garmin al respecte de la teva pregunta?

As the progression seems to hint on one of my Garmin 5K training activities: Run in Z4, threshold, 3 minutes. Run in Z4, 10K pace, 2 minutes. Run in Z4, 5K pace, 1 minute, It is obvious to me that when Garmin just asks you to run in Z4 (threshold) you can based it on how you feel that day with your restfulness, temperature of the day and/or how strong you feel. And for that you have all the range in your watch from 3.8-3.9 to 4.8-5.1. Yet if the excercise is obviously progressive like in this activity, it becomes obvious that Z4 threshold means the lower spectrum, let's say (3.8-4.1) and then Z4 10K  move to the very middle or (4.2-4.6) and finally Z4 5K take it to (4.7-5.1). Obviously you have to be in very good shape to be able to maintain 4.7-5.1, especially the highest part of this semi range during the whole duration of a 5 K race.