How much can I expect to improve during a 6 month training program? 
Most of the time goal runners I've worked with over the past 35 years have improved between 3% and 5% during a 6 month program, when they have done the long runs and speed sessions. If everything goes well in training and on raceday, a 15 to 30 second per mile improvement is possible. Weather and crowds can cause a slowdown in the finish time. (60F/14C or below is ideal). I usually allow my Ecoach clients to set a maximum goal of a 30 second improvement, if they wish. But if the magic mile is not showing this type of improvement as the goal approaches, I suggest attempting a goal no faster than predicted.

What is the longest distance of a long run, which will help me improve my time? To maximize endurance and performance, I suggest building the long one up to 29 miles. In surveys, I've found that there's an average of 11 minutes of time improvement when a runner increases the lengh of the last long training run from 26 miles to 29 miles. The pace of these long runs should be at least 2 min/mile slower than current marathon finish time—with the run-walk-run strategy appropriate to the pace. The last long run should be done 3-4 weeks before the marathon 

What type of speed training will help?
The type of speedwork that has helped my runners the most is a form of interval training called “mile repetitions”. The training format is detailed in Galloway Training Programs and Year Round Plan. Once the muscles are warmed up, each timed mile is run 30 seconds faster than goal pace, followed by a 5 minute rest interval of walking and/or jogging. Starting with 4 mile repetitions, the number is increased during each successive workout to a maximum of 14. These workouts are done on non-long-run weekends–in most cases the weekend after the long run. 

Are there any drills that can help me run faster, by improving my running form? 
I've discovered two drills that can help runners improve their running form. Both are done 4-8 times, at least once a week, on a short mileage day. 
• The cadence drill: Time yourself for 30 seconds. During this period, count how many times your left or right foot touches the ground (pick one before you start). On each successive thirty second segment, try to increase the count by 1 or 2. The goal is simply to increase the count by being more efficient
• The acceleration-glider: jog slowly for 10 steps, jog a little faster for @ 10 steps, then gradually increase speed to your magic mile pace for 10-15 steps and let your momentum carry you as you gradually slow down to a jog again (the glide). A 30-60 second jog or walk can be done between each. No sprinting. The goal is to learn how to use momentum to conserve resources. 

From Jeff Galloway's Marathon FAQ