Like you I want to improve my marathon time for brighton next year.... And my road half marathon time is about 2 hrs...
I have started doing some speed work once a week; I run up to the local school playing field and use their track. 400m (1 lap) as fast as I can go followed by a recovery lap repeat 4 times and then a gentle cools down run home I'm planning on increasing the number of repeats each week through the summer. I have an autumn half marathon booked that I also want to improve my time for.
Chatting to our club coach he has advised me to keep a long run of 13-16 miles in my schedule once every 2 weeks and then a shorter long run or race on the alternate weeks.
Personally I think to be in with a chance of a 4 hr marathon my half marathon time needs to be closer to 1:45... With a 2hr half marathon time I would hope to run the marathon in 4:30 ish.
I used the book "The non runners marathon trainer" to run my first marathon this year. It helps with the mental preparation as much as the physical. The longest run that I did was 18 miles (and only one of them due to injury). I finished the marathon and enjoyed every minute of it. It took me 5:10 including 20 mins spent queuing for the portaloos on the way round ! Even better I could still walk and manage stairs the next day
One thing they stress in the book is that for your first marathon your goal should simply be to complete it, having a time goal at this stage when you don't know how your body will cope with the training is quote possible setting yourself up for disappointment and taking the shine off of one of the greatest achievements in your life
I have kept a blog about my training and running here http://pancakemuddypaws.blogspot.com
From what I have read 12-13% body fat for a woman is bordering on dangerously low. Women do need more body fat than men and if it drops too low you will find yourself developing other problems.
To be honest I think that 19% for a woman of your age is low enough..... I would add have you tried circuit training in addition to running ? I'm a similar height and weight to you and currently have a body fat percentage of about 22-23% I have a few areas that I would like to improve on but just accept that we have no control over where our bodies decide to store fat or burn it from !