I have run Boston, New York and others. The main thing is to run about 4 times per week. This way you don't wear yourself out, especially if you are doing a lot of weight workouts. Make sure you do enough long runs. I add about 2 miles per week to my long runs and get up to 22 or 23 miles three to four weeks before. Make sure that you back off on your long runs the last few weeks. HAVE CONFIDENCE IN YOUR TRAINING AND MAKE SURE YOU TAPER! YOU DON'T WANT TO OVERTRAIN. During your long runs you don't have to run them very fast...I average 10 minute miles for my long runs which is about a one and a half minutes per mile slower than I run on race day. Make sure you carry water with you and put some out in a few different places to fill your bottle or change your bottle. I also make sure I walk 5 or 6 times for a couple of minutes (as Cathe would say a break and not a vacation) and drink at every mile so you don't get dehydrated. By taking the walking breaks you are getting a slight rest and you won't be completely worn out the next day. Also, take a cool bath when you finish for about 10 mins. It is tough but you will be happy that you did so the next day. I also have a Power Gel every 5 miles or so. Strawberry Banana is hte best flavor IMO. Your long run is the most important run of the week. Your other runs should be a 6 or 7 miler at a comfy but not too comfy pace, a shorter faster run, and maybe one with some hills. Also, make your long run on a course with some rolling hills if possible. The change in incline is better for your legs because they don't get pounded with 20 miles of the same thing and again your legs won't get as worn out. Just make sure you don't miss long runs. You can fool a 5K with not enough training but you cannot fool a marathon. Have fun. Thinking back to all of my training makes me want to run another one soon because I do all of my long runs with my dad and we have a blast together.
My suggestion would be to check out training schedules from 
