A 5K for fun is pretty much all fun. The real hardcore athletes are too busy running longer harder races so don't worry about it too much.
Most important things to do to prepare is extend your sprint time during your walks. This will actually help you increase your speed. I have a free 6 week
Interval Training Workout Plan that can help you with that.
Other things that can help you improve endurance so you can run at your desired speed the entire race are:
1. mobility exercises before your runs (this improves your stride)
2. stretching after your runs (prevents tight muscles that slow you down and pre-race injuries that are very common among ALL marathoners)
3. a clean healthy diet (key for efficient energy production)
4. the
Tabata Protocol increases endurance big time