Definitely call ahead and get registered for the mobility assistance. They can provide a wheelchair (if you don't have your own like my FIL does) and someone to push it (if you're travelling on your own or don't have someone in your party that can push). You also get to check in at a separate counter (usually with much shorter lines) and go through the wheelchair lines at customs and security with your Air Canada escort. They'll wheel you right to the gate...they've got the system down to an art form.