Strength training for runners includes running-specific exercises designed to increase your speed and power while running. This type of running-specific strength training becomes particularly important when you are trying for a personal best time or when you have a course with a lot of hills.
For weeks 5-8 of your 5K training, it is probably best to focus on running-specific resistance training movements. These will improve your body’s strength in general, but also increase your running power.
Perform 4-15 reps depending on how difficult the exercises are for you. 1-2 sets of each done twice per week is sufficient.
--Dumbbell Arm Swing
--Lunge with Medicine Ball
Again, for detailed descriptions of these exercises, see this page: Strength training for runners
You don't HAVE to perform specific strength training for runners, however. You can improve your running and your overall fitness by performing a classic full-body resistance training routine twice per week for 3-8 reps. Be sure to choose 2 exercises that are quad-dominant, 2 that are hamstring-dominant, 1-2 for the low back, 1-2 for the chest, 1-2 for the shoulders, and 2-3 for the abdominals. Click here for a classic strength training routine you can perform at home OR at the gym.
Whether you choose the strength training for runners or the classic strength training routine, it is most desirable for you to perform your strength training exercises during a separate session of exercise on a day that you do a less-taxing cardio session. They can also be done before your cardio sessions or on your "off" days from running.
If you perform a strength training workout and a cardio workout back-to-back, be sure to sip a sports drink (think Gatorade) or some diluted fruit juice, or at least have a good-sized meal a couple of hours before training. Stretching exercises are best performed after your running and/or resistance training. Focus your stretching on your calf, shins, hips, glutes, and low back.