Currently in this country, Cardiovascular Disease (CVD) is the single biggest killer, with around 275,000 deaths per year - nearly 40% (2 in 5) due to not doing enough activity. Most people, when asked why they don't do enough physical activity, say that cost and a lack of time and facilities are the main reasons.
Walking is free, can be built into everyday activities such as shopping, taking the children to school, going to work, etc., so therefore it doesn't need to take any extra time, and can be done from your own doorstep and thus doesn't need any special facilities.