Do short interval runs on Saturdays and long easy ones on Sundays. For Saturdays, do 2 to 5 minutes of intense intervals, with recovery breaks in between. Each session should not be too long, and should last 30 to 60 minutes. Start with something manageable. The number of repeats depends on your fitness level. Use Sundays to build stamina, and pace your long runs between easy and moderately hard ones, with each session lasting 60 to 90 minutes. However, if you only have time for one workout, go for the long, easy run.