There are three ways to determine and measure your "base" training pace range, which should be used for all training, except for speed work.

One is to base it on a recent race pace. Your training pace should be 1-2 minutes per mile slower than 10k race pace. That should be well within your aerobic zone. It can vary depending on a several factors, such as weather and how tired/stressed you might be. But, the 1-2 minute range allows for a lot of variation.

The second way is to use a heart rate (HR) monitor. Except for speed work, training runs should be in the range of 65-80% of your max heart rate with 70-75% representing a "base pace", below that an "easy pace" and above it a "brisk pace." Of course, this method means that you have to determine your max HR.

The third way, which is more subjective, is simply how it feels. A measure that can be used is the "talk test." As long as you can carry on a conversation while running, then you are satisfying the criteria of the above two methods. The lower you are in the above ranges, the easier it will be to carry on a conversation. At the upper ends of the ranges (brisk pace), which is a little too fast for daily training, you will have trouble talking in complete sentences, but you should still be able to converse.

Of course, with all three of these methods, the longer the run, the slower the pace will have to be in the base training pace range to satisfy the criteria. If your runs currently meet one or more of these methods, then you are "in the groove" with your training.