You'll want to start doing mile repeats once a week, about 8-10 weeks before your race. Here's what to do:
1. Go to a track where you can run a measured mile. Most tracks are 1/4 mile, so four laps would equal a mile. If you don't have access to a track, measure out a mile in your car or using a site such as MapMyRun. You can also do this workout on a treadmill.
2. Start with 2 mile repeats in the first session. Run each mile at about 10-15 seconds faster than your realistic goal marathon pace.
3. Recover (at an easy pace) for a half-mile (2 laps of the track) in between repeats. Make sure your breathing and heart rate have recovered before you start your next repeat.
4. Add another mile repeat the following week. Try to maintain that same pace (10-15 seconds faster than your realistic goal marathon pace) for each one.
5. If you're an advanced runner, try to work up and working your way up to 6 repeats. Intermediate runners may want to stop at 4 or 5 repeats.
Rules for Running on a Track
8 Rules for Speed Training