Very often as a casual runner-- someone who runs three or four times a week--you might want to challenge yourself by entering a race. At that point it would be wise to decide what you want to achieve.
Try to run a prescribed distance in a predetermined, realistic time--for instance, 10 kilometers in 42 minutes. Consider the running that you are currently doing. If you do comfortably four five-mile runs a week at about eight minutes a mile, you could strive for 10K in 46 minutes 30 seconds (a 7:30 pace for the mile). In any case, it is better to underestimate.
Having established the goal, continue your regular running schedule with one exception: two weeks before the race, run half your goal distance at race pace. Run as if you were in the first half of the race. If you don't run yourself into exhaustion, you can be confident of achieving your goal; if not, adjust to a slower time. If it is easier than you thought, you may want a faster goal time.
As you near race day, the last run should be easier than normal and only half the race distance. Take this run just to stay in your routine, not for additional work. Stay with your normal diet; carbohydrate-loading is not for this type of effort. On race day, rise early enough for your body to be "awake" by race time. Two or three hours before race time should be sufficient.
When the race begins, be careful to keep your initial pace well under control. Too many runners let their enthusiasm at the start of a race get the better of them and they run much too fast. It is better to be at race pace or slower early in the race.
Think of the first half as "setting the table" for the second half. Make sure you get to the halfway point ready to run, not physically spent.
On the day after the race, be sure to stretch and take an easy recovery run. Then stretch some more. With your next run, you can begin preparation for a new goal.