During marathon training, I run between 85-110 miles a week. More important than counting miles is monitoring how you feel with the work load. A high mileage program isn't a good idea if it makes you exhausted. Balance is the key. It's hard to tell what you might need.
Long runs are essential for good marathons.
If you train 12 weeks for a marathon, you might try starting at 10 miles for your long run the first week and build that up to 20 miles three weeks out from the race. After that you would taper the last one or two long runs to 12-14. I began to run 3 years ago and since I started I have gotten pretty good times. I had a really bad bout of injuries last year and missed 6 months, but I am in good condition now. At present I am running a 12.10 for 2 mile road race, but that is what I have been training for, now I need to move up in miles but maintain as much speed as I can.