I agree with what Cotton Shirt said, but would supplement it with a subjective test: after your warm up routine, start out at your estimated half marathon pace for you 20 minute tempo, or use about 20 seconds per mile slower than 5k pace as a rule of thumb. About 5 minutes in, test how hard your going by reciting the Pledge of Allegiance. If you can recite multiple clauses comfortably, you are going a bit too easy, so pick it up. If you can get out a clause, but need a breath before the next clause, you are about where you want to be. If you are struggling to get a clause out, or are panting hard after, slow it down a touch.
Retest every few minutes. Over time, you will not need to retest because you will learn what the right perceived effort feels like. The biggest danger of this approach is that you misjudge your pace early because it feels easy because you are fresh, or you slow down too much over the last mile of the effort because the run naturally gets harder as you progress over time at the same pace. But that should be a good rule of thumb to use as a starting point, and you will lock in the effort over time.
In my view, by the way, the 20 minute tempo run is the best bit of training you can do, regardless of distance, once you establish a baseline of fitness by running regular distance.