From: Daniel Duane, Fitness Crazed (edited)
Athletic coaches the world over know how to get people fit: every week for several months, take a few short runs midweek and a single long run on the weekend. Make sure the long run gets a little bit longer each time. Before you know it, you’ll be able to run 26.2 miles.
This works because the human body is an adaptation machine. If you force it to do something a little harder than it has had to do recently, it will respond — afterward, while you rest — by changing enough to be able to do that new hard task more comfortably next time.
This is known as the progressive overload principle. All athletic training involves manipulating that principle through small, steady increases in weight, speed, distance or whatever.
Ignore supposed research that tells you that you should do intense, brief workouts instead of long ones.