You don't have to join a gym to keep fit: going for a walk is an excellent, low-impact way to exercise. But just as you have to visit the gym regularly to see results, walking will only have a positive effect on fitness levels if you do it regularly (2.5 hours a week at a brisk pace is about right).
If you don't fancy pounding the pavements or walking three times round the local park every day, the answer is to live close to good walking country. The following properties all fit the bill perfectly.