The thing I find is that if I warm up slowly, I can run faster and longer. If I leave the driveway running, I'm gassed in a couple of miles, and it's all a struggle.
Works for me. In addition, I never run "on the clock" or "mileage". My whole run is predicated on how I feel. When I hit a comfortable pace (varies daily), I stay there. I have different routes for how I feel also.