Awesome defined......tempo runs! Don't worry, you'll be finished before your body evens knows you started.
Tempo runs are the single most important workout you can do to improve your speed for any race distance. Long runs develop pure endurance, but, tempo running is crucial to your race performance because it rains your body to sustain speed over distance. The problem being, as runners, we rain our cardiovascular system to deliver oxygen to the muscles but we have not trained our bodies to use that oxygen once its present. Imagine doing the tango without a dance partner. In essence, you are missing the crucial detail. Tempo runs do just that; they teach your body to utilize oxygen.
There are many ways of performing tempo runs, but, I prefer to use the progressive type and build on the distance. Its simple to perform and is in many ways more enjoyable as your body gradually warms up to your pace. You feel stronger through the middle portion and can typically finish at a faster pace. To do a progressive tempo, start with one kilometer (1/3 your distance) at warm up pace. Then gradually and evenly increase your pace over the next kilometer or second part of your distance at an easy endurance pace (60-70% of your maximum heart rate-you can hold a conversation). The last kilometer or final 1/3 of your distance is at race pace (90% maximum heart rate - just shy of breathlessness).
This type of tempo run can also be divided by time rather than distance. It gives you the additional advantages of running while improving your ability. It will also help with those negative splits. You'll be running faster at the end of a long quality run.
Note: Ensure that all speed work sessions begin with a dynamic warm-up and cool down routine. And, don't forget to stretch.