With your base you should be fine. When I did my first marathon 4 years ago I had little experience and had only planned on doing the half. A month into training I realized I could go the distance and did the full with no problem. I only ran about 20 miles a week because of bad knees and I had a great race, but I wasn't looking to qualify for Boston or anything either. You have a good base, so you should be just fine. The long runs will come with time and training. Just pick a good program and only run what it tells you to and take time off when it tells you to. www.halhigdon.com has good plans. I always follow his when marathon training (I've done 6 now).