I'm on week four and it's been tough, but a great and free way to improve my fitness.
*UPDATE* - I've just finished the programme (working towards doing the Race For Life) and I feel great - never knew I'd be a runner - couldn't recommend it enough
You can download the podcasts for free and 'Laura' will guide you through your runs.
Good parks to run around include Eaton, Wensum and Waterloo Park, or Mousehold
Couch to 5K Website
More ways to get fit for free