My first marathon was 2:43 with nothing like your mileage. Your 20 mile runs at 6:30 pace are very, very impressive although they are so close to a race effort, I'd worry about the injury risk. At this point is really too late to introduce any different workouts My only real advice would be to resist any temptation to run faster early and to take weather into account if it's hot. Your 2:40 target is 6:05 pace - so the way to think of it is if you allow yourself to drop below 6:00 pace for even one mile, it will cost you 10X in time later in the race. When you "bonk" and can barely hold 8:00 miles it is incredible how much it costs you, even if it is only for 2 miles