Two-a-Day Principles. If you try two-a-days, keep these seven guidelines in mind:

1. Separate your workouts by at least 2 hours; 4 hours is even better.

2. Be sure your first run of the day is very easy. Save any speed sessions for later in the day.

3. Don't divide your long run into two shorter runs. Continue to do your long run all at once-that's what makes it a long run.

4. Warm up for your second run of the day, as you did for your first.

5. If you're training for a race, at the end of your morning run do three to six accelerations of 50 to 100 meters each (walk or jog 100 meters in between to recover). Run them at your race pace.

6. Eat breakfast (or a high-carbohydrate snack) within 30 minutes of finishing your morning run.

7. Increase your mileage by no more than 1 mile per run. If you increase your weekly mileage more than 3 weeks in a row, don't raise your total by more than 10 percent a week.