First, your keyframes show the path is below the floor. Second, you have 3 and 2 would do. Third, with the CTI (Current time indicator) set to the last keyframe, select the pen tool, hold down the Alt/Option key and then hover over the keyframe with bezier handles and you should see the convert vertex tool. It looks like a little V turned on it's side. Click not the keyframe to clip the bezier handles and change the spatial interpretation to linear.
Please read up on animating position in the help files. All of this and more is explained. These are basic AE skills you need to master: Have you gone through these getting started materials? Basic Workflow