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.

ANSWER
In 28 weeks, you could finish a marathon. Start by walking, slowly, for a mile. Two days later, walk 1.5 miles. Every other day, increase the distance by half a mile until you get to 3 miles. Rest your muscles on the "off" day. Hold that distance on Tuesday and Thursday, while increasing by one additional mile on your weekend session (either Sat or Sun). There's a "to finish" schedule in my book MARATHON YOU CAN DO IT. Once the long run reaches 10 miles you can cut the distance in half every other weekend, increasing the long one by 2 miles each time. When the long one gets to 18 miles, you can do it every third weekend, covering 8-9 miles on the "short" weekends. After about the third week of training, insert a one min jog into your runs, after 4 min of walking. If this feels fine after 2 weeks, drop to 3walk/1jog. After 3 more weeks, drop to 2-1. If all goes well after another 3 weeks or so you could go to 1-1 during the week, and 2walk/1jog on the long run. Find the ratio that works best for you, that will leave you feeling strong at the end, and recovering fast from these long ones. The pace for all of these should be very, very slow.