If you're interested in running a marathon, an important thing to do is make sure you train according to your lactate threshold. This is the fastest you can run (with the same speed from start to finish) without feeling any burn or pain (which is generally caused by the lactic acid that accumulates in your legs when you exert a lot of energy). If you're training as close to your lactate threshold as possible, you're quite simply getting the most from your run. A quick tip to do this? Say the 31-word Pledge of Allegiance out loud as you run. If you're working at your maximum capacity, you should get through it, but only in bursts of three or four words at a time.