Hello Sally and welcome to the forum.
I'm glad to hear that you were bitten by the bug following completion of your first 5K; I'm sure that being bitten has served you well, both in terms of fitness and weight loss.
However, marathon training is a wholly different ball game, even taking into consideration the training needed to complete a half. Since it'll be your first full marathon, the best advice I can offer would be to ensure you've put in sufficient training (which I'm sure you have).
Provided you've done so, your half marathon time should provide you with a pretty good indication of how long it'll take you to complete the course.
Your longest run should take place at least three weeks before the event, providing sufficient time for recovery afterwards, but don't worry if it isn't quite marathon distance. If you're able to cover 22 miles, you can cover 26 (crowd participation carries you around the course).
Regardless of your recorded time on the day, consider it an incredible achievement, Sally. Don't worry if you feel emotional after its completion, either, since you'll have achieved a monumental feat. Moreover, the time you record on completion is your marathon PB; one to be beaten in subsequent marathons.
As for training, although it's heavily geared towards increasing distance, don't dismiss the benefit of more intense sessions, not least those that exercise the body's anaerobic system, since they'll have far reaching benefits upon your ability to increase distance. Intense sessions don't necessarily need to be performed outdoors either, thus, providing your joints with a restful break from repeatedly pounding the pavement (runners in the know cross-train).
Take to the gym and perform intense intervals of 60s, followed by 30s recovery upon machines such as the spinning bike and Stairmaster, for example. Provided the hard intervals push your body into its anaerobic zone, not only will you increase the ATP density of your muscles (more power), you'll improve the mitochondrial activity in your heart, whilst also increasing your body's ability to handle increased lactate concentration in the blood.
Given that increasing distance is your current goal, you'll be relieved to learn that a HIIT session lasts no longer than 20-25 minutes, making it something to look forward to. HIIT is also a great way of maintaining your fitness during your tapering period, too,
In short, the physiological adaptations of HIIT benefit endurance activity, since the heart and muscles become less reliant upon processing oxygen and far more efficient at handling and utilising rising lactate levels during steady state cardio, something that'll help you in the latter stages (you know what I speak of, Sally).
Most of all, enjoy your training, since it's something that'll see you toe-ing the starting line with confidence come race day.
