I'm sure you'll not be long before you are knocking those out in that time. I found once I was able to get below 5/km, it was a lot easier to get down to 4:45/km and then now at 4:30/km. It feels weird running slower now, but it's hard keeping 4:36 for a half marathon. I basically need to get down now to 4:06/KM to get down to 41 minutes for the 10KM at the end of September.
I need to do some more shorter distance speed work, and mix it up with some interval middle distance stuff.