PR Principle #3: Specificity
While more advanced runners are likely running workouts every week, to truly reach your potential and run a new personal best, it's important that the workouts during your peak training phase are specific to the demands of your goal race.
In this case, the peak workouts must mimic what you'll experience during your 5K on race day. Sure, 400m intervals have their place in a good 5K training program, but they're not as specific to the demands of the race.
The following two workouts should be completed on a track so you can monitor your split times more accurately.
The first workout is five repetitions of one-kilometer intervals done at your goal 5K race pace, with one minute of easy running in between each, or 5 x 1K @ Goal 5K Pace, 1' jog recovery.
The second workout is three repetitions of one-mile intervals at your goal 5K race pace, with one minute of easy running in between each, or 5 x 1 mile @ Goal 5K Pace, 1' jog recovery. It's slightly more advanced because of the longer intervals.
Both workouts are very similar to a 5K race; they challenge the body in similar ways. If you can complete either of these, then you're ready to set a new personal best.
Consistent improvement in any race can be traced back to smart training, and a good progression of workouts and training stimuli. Keep taking the next logical step in your mileage, long runs and workouts and you'll continue improving for years.5K.