It seems to me your current mileage is fine. My target is 3:15, and at the moment I am doing around 30-35 miles a week, with a longest run of 10, aiming to get up to around 15/16 by the end of the month.
I normally try to do 4 or 5 20 milers; the first one at a very easy pace (anything from 2:45 up to 3:00), the second one a bit faster, and then a couple of 20 mile races at marathon pace.
The only problem with that strategy this year was that having done a couple of 20 mile races at 7:15 min/mile pace, I decided that 3:30 was too conservative a target, and spent most of the first half of the marathon ahead of the 3:15 pace group. I ended up walking large parts of the last 10 miles, and finished in 3:38. I think the lesson is to let your 20 milers give you confidence, not over-confidence. Next year I am not going to make the same mistake.