I love Australian Shepherds! They're one of my favorite breeds of dogs, and they are intelligent, loyal, loving companions. I grew up with all kinds of herding dogs, and they have forever stolen a part of my heart - I'll always have a least one with me.
My Schatzi is a border collie/aussie mix of some sort, and my heart dog Nellie was an aussie/golden mix. An aussie and a lab should complement each other well; Schatzi and Pickles get along great. Unfortunately, many aussies are in rescues and shelters because of their high energy needs, but they are very smart and trainable, and your long walks & off-leash activity sound perfect. Good luck!