Running is tough. You will get tired. You will develop aches and pains. You probably won't experience the vaunted but mostly mystical 'runner's high.' But none of these are sufficient reasons to quit your training program. In fact, virtually no reason is sufficient. You'll miss days, you'll catch a cold, you'll have a long plane flight, your father will be hospitalized, and so on. Life is full of events we can't control, and these events will temporarily derail you from your best-laid plans. But don't quit. Get back on track as soon as you can. The lost time won't hurt you. Only quitting can do that.