Ask The Experts

Answer:
I see limited benefit from running twice a day during a marathon training
program. The most important element in marathon training is the long run.
As you build it to 26 miles or farther, you decrease your chance of hitting
the wall. But the long ones need to be run continuously. Breaking the
distance up into two runs decreases the training effect.

Running twice a day can increase fat burning, while reducing the pounding of
running on short runs during the week.

- Jeff Galloway