Long run/Training debate

I have a question I am pondering. I am training for my first marathon which is next September, a long time from now. But before that race, I will be doing 2 half marathons in preparation. Should my training be focused on my marathon and getting up to 20+ miles on my long run, or should I be doing a half marathon training and then increasing my long runs a certain time before the full marathon in September? Currently, I am doing around 25-35 miles a week, and my long run has increased to 15.68 miles. Advice?