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.
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).