What I've discovered thus far (for myself at least) is that the first mile is definitely the hardest. Each mile after that it gets better (easier?), and by the time I get to the 4th mile I really feel "warmed up." That's when I know that I can just keep on going, and don't ever want to stop. And THAT is what makes me truly believe that I am really a runner- I don't ever want to stop!
And I'm still amazed that my feet don't hurt!