How hard should you run? It's a very open ended question, so the answer is "it depends"!! However, for most of your runs, most of the time, you should be running at an easy pace. How easy? Easy enough that you can talk in full sentences if someone is running with you. Not gasping and stuttering a few words at a time, but being able to have a good conversation. This pace corresponds to the Yellow Zone intensity in our training sessions, and should account for somewhere in the range of 80% of your training time.
So next time you're out for an easy run, ignore your GPS pace and just run to feel - some days it will be a little faster, some days a little slower. Whatever. Just enjoy the comfort of an easy run, and save the high(er) intensity running for other days.