The first key is to schedule a long run every week. The long run will fall on the weekend for most people since that is when they have most time to devote to running. Let's assume you'll do your long run on Saturday morning. The long run will help you develop your aerobic metabolism, which is important since the 5K is over 90 percent aerobic in terms of the energy contribution needed to run the race. Plus, if you want to move up to 10Ks, half marathons and marathons, the long run will be central to training for those distances, so it's good to get into the habit of running a solid weekly long run now.
Your other weekly workout, completed ideally on Tuesday, will focus on increasing your speed. The workout will alternate between a purely aerobic run and a session where you run at race pace. See Step 5 for more specifics on 5K workouts.
And this brings us back to the importance of that baseline 5K race. Once you run the baseline race and know your pace, you can use that information in training. You can do your 12 x 400m workout (see 5K Race Pace under Step 5) starting at your baseline race pace, but then get gradually faster on the 10th, 11th and 12th repetitions of that workout.
5K Training Plan Step 5: Workouts
Beginners should start with a fartlek workout for the first Tuesday aerobic workout. Fartlek is a Swedish term that means "speed play," and that's what it is—you're playing with two difference paces during the workout. During the "on" portion, you'll run a challenging, but very doable pace for three minutes, then, with no break, you'll go right into your "steady" running portion, for two minutes. Steady means that you should run faster than your normal easy day pace, yet slower than the challenging "on" portion. In this example, you have a five-minute block: three minutes on and two minutes steady. Do that block five times and you'll have 25 minutes of fartlek. Warm up for 10 minutes, do 25 minutes of fartlek, then cool down for five minutes, and you'll have a great 45-minute workout.
2. Progression Run
Another good aerobic workout is a progression run. You could do this workout by miles. The first mile is your warm-up mile. The next 2 miles should be run progressively faster, starting at a pace that is challenging, but that you're able to maintain. Gradually pick up the pace over the next 2 miles so that by the end of the run, you can say, "I could have gone another five minutes at that pace, but it would have felt like a race." Go straight into your 1-mile cooldown. It might make sense to split that 2-mile progression run portion into four half-mile segments, making each one a bit faster.