OK first the scary bit. This is serious and as someone who is genetically gifted for marathons I have witnessed the ambulances first hand.
Marathons can and do kill people every year, treat this event with the utmost respect and realistic fear. If you are in any doubt back out even if part way through.
You have a start and that is good, but trust me the difference between half and full marathon is not the same as 2 to 4 miles or most other double ups. The last miles of a marathon will pull from resources the body is not used to unlocking and failure to be ready can result in exhaustion, hypoglcemia (hitting the wall), sever dehydration and any or combinations of these can lead to hospitalisation and in extreme cases death.
Doom and gloom over training guidance.
First and foremost, run further and more frequently but ensure you get the rest you require.
If you want to be ready for summer, not sure when that is there, and are used to 2 and 4 mile runs now and are 15 years of age, forget a quick time, just aim to finish this time and improve in the future. As such intervals etc. will not be a requirement, unless you find yourself doing incredibly well and can ad this safely.
Make sure your technique is efficient and relaxed, this will help avoid injuries and enable you to run further without stopping.
Initially run distances you are comfortable with multiple times a day so if you have just moved from 2 to 4 miles, drop back to 3 and do 2 runs in the day.
Increase the distance of these gradually. This will be 2 days on 1 off at first. This will get you used to feeling the effects of long periods of high impact work with minimal risk. If you are having problems come back and ask for help.
Next step will be to start adding at least one long run in a week, so the program could be as below for the week, dependant on the distance you are doing when feeling ready for this.
Day 1 2 x 5 miles
Day 2 2 x 5 miles
Day 3 rest
Day 4 2 x 5 miles
Day 5 rest
Day 6 1 x10 miles
Day 7 2 x 4 miles
Again to increase intensity and familiarity with running distance, increase distance gradually and swap out more days with 2 shorter runs with one long one. don't just do long runs however, most should be 3 or 4 a week.
Milestone
Find a tough 18 mile route, or ideally an 18 mile event. This will help you prepare for a marathon like nothing else. Most people hit the wall close to this mark so a tough run of this distance will risk this but ideally nothing more than minor case. It will help you know if the marathon is realistic, or if you need to wait a while.
The women I knew when doing distance running were generally with other endurance athletes, they are the only people who could handle losing them for such long periods or train with them. You will live eat, sleep and breath for the event, this is why so few do it.
Any figure you have will disappear, marathon runners have a shape, but it is one that hides behind lamposts. They are muscular but it is the wiry barely visible under clothing type.
Final note. Good luck.
