HAL’S ANSWER: The moment marathon race directors allow you to run 13.1 in the morning, then 13.1 in the afternoon after a lunch break, subtracting the delay from your finishing time, then sure, you can break your long runs in half. But seriously, it doesn’t work that way. Yes, there are some benefits to be had from the total mileage run in a single day, but not as much as if that mileage came in a single workout. The most important benefit is the ability to run for 15 or more miles without stop. Breaking the workout in half defeats that purpose. I might even suggest that the benefits in a four-hour run don’t come until after you have run three or more of those hours. Only then will you begin to experience the fatigue felt in the closing miles. Doing a long run of whatever distance conditions you for the physical challenge you will encounter in the marathon itself.