As you start running longer distances to train for a marathon, pay attention to your muscles, your feet, and how your body responds in different temperatures, and after different distances. When you increase your mileage, decrease your pace so you can run the entire distance at approximately the same speed. If you are used to sprinting for 3 miles for a workout, you will need to moderate your pace as you adjust to a longer distance. Your easy runs should be easy enough for you to carry on a conversation with your fellow runners. Not only does a “conversational pace” ensure that you aren't going too fast, it makes the run more fun.