Hi cazz
Welcome to the forum
You have got ages to build up, so you don't need to stress too much about mileage right now, so long as you are running regularly and so getting your body used to it being a regular thing.
What you don't want is to do too much now and pick up an injury, so listen to your body carefully. It will tell you if you are doing too much.
What you need to do is find a proper marathon schedule, one that you can fit into your lifestyle without too much hassle. You can get them that last up to about 20 weeks, so pick a fairly long one. There are plenty around, there are some good ones on Runners World, plus in most running books. Hal Higdon also has good schedules. You could probably google 'marathon schedules' or something.
Then plan it so the schedule ends on race day...put it in a diary or something. Now you will have a schedule start date and something to aim for.
Up until the official start date of you schedule keep training regularly so you are in the best shape to start the schedule.
Keep posting, there are plenty of people on here with some good advice that will help you through your first marathon training and the race itself
