I have started the Couch-to-5k running plan. I have just started week 4, and I can't say enough good things about the program. I run for time and use my ipod to keep track of the intervals. The program is challenging, but not impossible (one of the things that makes it a good fit for me). The plan assumes that you will run 3 days a week and each day's run is only about 20 minutes.
I really like the structure of the weekly intervals, it pushes me just enough that I know I'm improving, but not to much that I just get overwhelmed and quit. Because the intervals change each week there is some variety in the program.
If you are looking for something to help you start an exercise routine or you are a frustrated runner, check this out.