If you go from “just finishing” a race distance to “how fast do I have to run” to beat a time of X, then you want to check out the Running Times Magazine’s Running Times Pace Charts. They have charts in both miles and kilometers, and cover the most popular race lengths.
Fall is coming soon, and with that will come long distance races. As I am getting ready to run a half-marathon, I will be definely checking it out. You might want to as well.