Walking is probably the safest way to start off your fitness programme if you have not been exercising or doing sports for a long time. Many people have found success with keeping fit by walking. It's cheap, easy to do and requires very little preparation. A healthy walk is defined as 'a purposeful, brisk walk undertaken on a regular basis'. The key word here is 'regular'. You will only see significant effects if the walk is done regularly every week. Start off with 15 to 20 minutes walk, two or three times a week. Gradually increase the duration to 30 or 40 minutes each time.
The idea is to enjoy the walk so that you are motivated to continue with it for a long time and not for just a week or two. Be creative - change your route or find a walking buddy. This way each will be able to check and motivate the other or you can join a walking/ramblers club.
Below are some links to websites with more information about walking:
1. Walking the way to health
2. Walking to Fitness
3. Walking.org