Activities are logged with Garmin devices like watches and bike computers. When you sync the device with Garmin Connect the activity will be added to My Day.
What are you adding? Do you manually create an activity? Why not use a Garmin watch and log your walk?
Ok. The only way is to start an activity on the watch. Use some kind of trigger to make you remember. After a while it will be automatic for you and you will start an activity every time.