Click TRAINING, then TRAINING PLANS, then look at several of the training plans posted. There are a couple that marathon-specific.
Most marathon training
workouts include one long & slow run per week. That distance normally increases as you get closer to the marathon date. In your case, the 13-miler is your long & slow run. It doesn't appear to be long enough using conventional marathon training ideas.
Jeff Galloway's training workouts for marathon training incorporate the run-walk-run protocol. Run for say 5 minutes, then walk for 1 minute and continue. Using this method, Jeff advocates covering a greater distance than if you just ran on your long & slow run day. This method might work for you since I really believe your long & slow run day needs to cover a greater distance.
I've never ran a marathon, only a 30k many years ago. But I talk to people regularly who are training for marathons and I know what type of training they are doing.