First, welcome to the blog.
I looked up your race results and tried to look up your training, although I really don't know how up-to-date your FRB is in terms of training. If you aren't already, the first thing is simply to increase your mileage. I think the highest mileage you showed was 32 miles in a week, and 20 of that was one run. If that really is your training, I think a 3:30 is quite good (35 miles per week usually results in bad bonk at mile 18 or so).
To improve, there are no secrets. Run. A lot. You need to consistently run at least 5-6 days a week. Running 2-3 days per week will never get you in good marathon shape, despite what Runner's World may claim. 80-90% of it should be easy. If you aren't running at least 30 min per day, 5-6 days per week, I don't think you should do any speedwork. In fact, I would suggest 45 min or more per day, or even an hour. Then, you need lots of long runs- I would recommend at least 3-5 long runs of 18-22 miles pre-marathon. Finally, race more. More half marathons will really help with a marathon, especially if you do it as part of a 20 mile day.
If your dedication, family, and body allow it, try to build up to 50-60 miles per week and maintain this for 3-6 months. If I mis-interpreted your training, please let me know.
Here's a few other posts to read:http://fastrunningblog.com/forum/index.php/topic,1546.0.htmlhttp://fastrunningblog.com/forum/index.php/topic,1360.0.htmlhttp://fastrunningblog.com/forum/index.php/topic,1455.0.html
Good luck and feel free to ask more questions. We love to offer advice, probably moreso than people want to hear.