You really can get fit just by walking, but you must walk long enough. A recent study show that people who are success maintain their health just walking as their main physical activity.
Walking is easy activity, every people can do it and didn’t need money. Maybe walking burns fewer calories per minute than jogging, but most people can walk last longer than a run. Walkers also relatively low injury rate than runners.
If you really didn’t have enough time to walking on the park, you can do it at your daily activity. For example you can parking your car far away from your office, so you will walking longer to your office.