You should ease into your running program 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 running three miles (or 5K) 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.
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.
There are two ways to follow this 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.