Taking this race duplication stuff a little further, I began to lengthen out the hard efforts, eventually to about 20 minutes in duration. This would be done once or twice a week, depending on how I felt. Well, now it's 1994 and everything has an official name. The people who make a living out of studying this kind of thing call this Anerobic Threshold Level Trianing. The term refers to training at a level just below the point where lactic acid begins flooding into your muscles.
Lactic acid acccumulates when a muscle is overworked.
It causes the muscle to lose efficiency, strength, and flexibility. If you are going to try training at Anerobic Threshold Level, you need to get a good idea of what it is. If you're not into high tech, try starting at a percieved effort level of about 10-15 seconds per mile slower than your 10K race pace. In either case, listen closely to your body in the 2 days following your workout and slow down if need be. With patience and consistency you can improve performance without sacrificing the fun and spontaneity that running brings to us each day.