A recent run I did with a mate provides a good example of what I'm talking about, his goal was to hit 12ish minute miles for a 16 miler. We ended up with a 10 minute mile average, success right? NO! That was not the goal for the pacing so we failed even though it was a very enjoyable run. Now keep in mind he has been running for years, is one of the fastest runners in our club, and a total beast when it comes to fell races of all distances, long and short so hopefully that will put it in perspective how important and difficult it is to learn how to pace across all speeds. Going slower in my opinion quite often requires more discipline than trying to run fast.
Learning how to pace is not only about keeping a certain average per mile but learning HOW that speed feels and being able to adapt it for various distances. I realise this is somewhat abstract but practising hitting all speeds (slow, medium and fast) means that your muscle memory will kick in automatically and you no longer have to rely so heavily on your watch to monitor it. Smarter pacing=better racing!
Pic via popsugar.com