when training for a race, there are seveal phases to the running plan. It begins with a base building phase followed by a sharpening phase and lastly a taper phase. Basebuilding usually is about 75% of the total training plan time with sharpening the next 20% and finally the taper just before the race (whether running 5K, 20K or 42K). So for a typical 16 week training cycle, 10-12 weeks are devoted to base building, 3-5 to sharpening and 1 or 2 to taper (if a 5K runner, taper is usually measured in days not weeks).

The base building is the slow increase of weekly mileage and increase in the length of each individual run within the weeks. THis builds your aerobic fitness.

Sharpening phase is where we start to work on going faster. There are several different types of runs, of which the pace run is one of them. In a pace run, you run at or near the pace you want to run for your competition. THe distance you run is not the same distance you will run for the competition but it is intended to get you to know what it feels like to run at that pace and helps your body do the physiological things it must to keep you running at that pace.

Hope this helps.