Extending the length of a path

I know paths are useful, but I’ve started to avoid using them because I don’t know how to extend them. I know how to make a path longer, by editing the end of the IPO, but just changing the length can disturb the relationship between points on the path and other events. For example, maybe I have a path of length 100, with a camera following it. At frame 87, say, the camera is at a certain point in space. How can I extend the path to length 120 and still have the camera at the same point at frame 87?