Your intuition brought you to Susan. Intuition is knowing something without reasoning or explanation. I assume that you have had many heart to heart talks with Susan, which in essence has created a strong bond in your friendship. I believe you sensed her presence and she may as well sensed yours. Being the intuitive people you are, both of you followed the force until you both found the source...each other. 

Looking at the situation from a different perspective, such as why did you decide to take a walk the same day Susan needed a friend? The answer lies in "Divine Intervention," which is the term I created for such instances. Divine Intervention got both of you out of the house for a walk, but intuition brought you together.