Goal - Work on marathon pace

Workout ( I am going to use a 5K race pace of 8 min and goal marathon race pace
of 10 min miles because the math is easy)

Workout is a five mile run on the track as follows:
Brief warm-up
First lap at marathon pace (2:30 min or 150 sec)
Second lap at 5k race pace (2:00 min or 120 sec)
Third lap at marathon pace (2:30 min or 150 sec)
Fourth lap at 5k race pace (2:00 min or 120 sec)

and continue. Each odd number lap at target marathon pace, and each even
numbered lap at 5K race pace.

The key: Run your recovery laps at goal marathon pace, not faster, not slower.
If you run your fast laps too fast or too slow, that is OK as long as they are
faster than your marathon pace and you are able to regain your target marathon
pace.

The workout actually goes much faster than it sounds due to the desire to
recover after the fast lap and the need to push it hard during the fast lap, but
your success is predicated on the ability to hit your target pace on the slow
laps, not the fast pace on the fast laps!