Check out this great article from Cool Running on how to train for your first 5K Run!
First off, you may be asking, “exactly how many miles is a 5K?” A 5K is 3.1 miles. We’re here to help you achieve your goal of beginning to run your first 5K. Too many people have been turned off of running simply by trying to start off too fast. Their bodies rebel, and they wind up miserable, wondering why anyone would possibly want to run in the first place.
You should ease into your 5K training plan gradually. In fact, the beginners’ program we outline here is less of a running regimen than a walking and jogging program. The idea is to transform you from couch potato to runner, getting you to begin running a 5K or 3.1 miles and on a regular basis in just two months.
It’s easy to get impatient, and you may feel tempted to skip ahead in the program, but hold yourself back. Don’t try to do more, even if you feel you can. If, on the other hand, you find the program too strenuous, just stretch it out. Don’t feel pressured to continue faster than you’re able. Repeat weeks if needed and move ahead only when you feel you’re ready.
Start Your 5K Training  A Few Minutes Each Week
Each session should take about 20 or 30 minutes, three times a week. That just happens to be the same amount of moderate exercise recommended by numerous studies for optimum fitness. This program will get you fit. (Runners who do more than this amount are doing it for more than fitness, and before long you might find yourself doing the same as well).
Be sure to space out these three days throughout the week to give yourself a chance to rest and recover between efforts. And don’t worry about how fast you’re going. Running faster can wait until your bones are stronger and your body is fitter. For now focus on gradually increasing the time or distance you run.
Run for time or run for distance
There are two ways to follow this Couchto5K training program, to measure your runs by time or by distance. Either one works just as well, choose the option that seems easiest for you to keep track of. If you go with the distance option, and you are not using a track to measure the distances, just estimate. It’s not important to have the distances absolutely exact.
NOTE: When using the mobile app, you must follow the plan by measuring your runs by time. Although it tracks your total distance with GPS, you cannot measure your runs by distance on the mobile app.
Before setting out, make sure to precede each training session with a fiveminute warmup walk or jog. Be sure to stretch both before and after.
5K Training Schedule
Bookmark this Couchto5K running page so that you can easily return to check on your progress.
Week  Workout 1  Workout 2  Workout 3 
1  Brisk fiveminute warmup walk. Then alternate 60 seconds of jogging and 90 seconds of walking for a total of 20 minutes.  Brisk fiveminute warmup walk. Then alternate 60 seconds of jogging and 90 seconds of walking for a total of 20 minutes.  Brisk fiveminute warmup walk. Then alternate 60 seconds of jogging and 90 seconds of walking for a total of 20 minutes. 
2  Brisk fiveminute warmup walk. Then alternate 90 seconds of jogging and two minutes of walking for a total of 20 minutes.  Brisk fiveminute warmup walk. Then alternate 90 seconds of jogging and two minutes of walking for a total of 20 minutes.  Brisk fiveminute warmup walk. Then alternate 90 seconds of jogging and two minutes of walking for a total of 20 minutes. 
3  Brisk fiveminute warmup walk, then do two repetitions of the following:

Brisk fiveminute warmup walk, then do two repetitions of the following:

Brisk fiveminute warmup walk, then do two repetitions of the following:

4  Brisk fiveminute warmup walk, then:

Brisk fiveminute warmup walk, then:

Brisk fiveminute warmup walk, then:

5  Brisk fiveminute warmup walk, then:

Brisk fiveminute warmup walk, then:

Brisk fiveminute warmup walk, then jog two miles (or 20 minutes) with no walking. 
6  Brisk fiveminute warmup walk, then:

Brisk fiveminute warmup walk, then:

Brisk fiveminute warmup walk, then jog 21/4 miles (or 22 minutes) with no walking. 
7  Brisk fiveminute warmup walk, then jog 2.5 miles (or 25 minutes).  Brisk fiveminute warmup walk, then jog 2.5 miles (or 25 minutes).  Brisk fiveminute warmup walk, then jog 2.5 miles (or 25 minutes). 
8  Brisk fiveminute warmup walk, then jog 2.75 miles (or 28 minutes).  Brisk fiveminute warmup walk, then jog 2.75 miles (or 28 minutes).  Brisk fiveminute warmup walk, then jog 2.75 miles (or 28 minutes). 
9  Brisk fiveminute warmup walk, then jog 3 miles (or 30 minutes).  Brisk fiveminute warmup walk, then jog 3 miles (or 30 minutes).  The final workout! Congratulations! Brisk fiveminute warmup walk, then jog 3 miles (or 30 minutes). 