This article is from the Running FAQ, by Ozzie Gontang with numerous contributions by others.
Most running programs, ask you to run three times a week as a minimum
requirement. This helps reinforce the habit of running, but its main
purpose is to develop cardiovascular conditioning through frequent running.
But more is not necessarily better. Experts in physical fitness tend to
agree that running days should alternate with days of rest, since rest for
the body is as much a part of developing fitness as exercise.