If you go out in two or three days and try again, and keep on doing that, then you'll get better, and sooner or later you'll do eight runs out of eight.
Get Running lets you do weeks over until you're confident enough to move onto the next week (if it ever jumps you ahead to a week where you're not comfortable yet, just tap on the previous week in the little path down at the bottom of the home screen, and then when you press "run" it'll go back and let you re-run that week instead.)
When I did the Get Running plan, I started the "nine-week" plan in July, and finished it in October. Since then I've done six half marathons. And plenty of people have taken longer to finish the plan. Don't worry if you can't do every run first time, is what I'm saying