Dunners, great question! It depends on your appetite for risk and your self-awareness. If the gap between your current estimated fitness and desired fitness is large, now is the time to find out.
If you are not confident in your level of current fitness, use your current estimated pace. If you feeling strong and willing to take a bit of a risk, then go with your target pace.
The risk being that if you go with target pace and blow up, you’ve lost a critical workout.
And, a common cause of half marathon failure is not accepting actual current fitness and forcing a goal pace.
Is it realistic for you to hit 3x3km at target pace? I don’t know, but you’re about to find out!
You can also split the differnet: first 3km at current estimated, if that goes well, next at target pace. If that goes well, one more at target.
David