TT, who knows how well anyone will do on marathon day itself. When did you run your half marathon? A better indication would be to run another half marathon about 6 weeks before you next marathon. You may find as you increase you training mileage each week and do longer runs of over 13.1 miles your half marathon time will come down naturally. But if this is your first marathon the main aim is to get round in one piece and enjoy the whole experience, but aim all your training towards the sub 5 hours goal.