For endurance training, distance is more important than pace. You will find many programs will ask you to run slower for longer and run at a certain % of your anaerobic threshold.
"For example, if you’re training for a 10k (6.2 miles), make your shorter endurance runs four to five miles and longer ones seven to nine miles."
If the goal is endurance, you should not be running high intensity intervals which is what it looks like from your heart rate rising and falling. For endurance, you run one pace for a long time. So yes if you want to improve endurance, you have to slow down and quit intervals.
This website has tables to break down which % anaerobic threshold (based on 3 speeds) you can run in and how long you should spend at that threshold with respect to your training run.
*I plugged in an anaerobic threshold of 150 (a random # just to explain the website screenshot below) and race time to populate the numbers
This is how the website generates it's numbers:
· By endurance training you run in the same speed, therefore without discontinuances and speed changes.
· The mentioned % are percentages of the anaerobic threshold.
· The anaerobic threshold is approximately = 220 - age - 15. Another method is: take 80 % (beginners) to 90 % (advanced) of the maximum heart rate.
· Speed 1:
The long quiet endurance training: 75% and 80%
Convalescence training: under the 75%
· Speed 2:
The average endurance training: 85% and 90%
· Speed 3:
The intensive endurance training: 93% and 95%
Only arranged for the advanced runner in the preparation period.
So for example from the table below, if your run is 10 miles and you're training at speed 1, you will run it for an hour and 28 minutes at 75-80% of your anaerobic threshold. Maintain your HR around 128-136.
This website also has it in kilometers for those who prefer the metric system.