I believe that I have discovered the single best, and by best I mean most bang-for-your-buck, workout a 5k/10k can do, really at anytime throughout their season.
It is similar to Pre's 30/40 workout. I call them "Threshold-Fartlek Intervals". You take the runner's LT pace, the pace used for a standard 4 mile tempo run (LT) or tempo intervals and add/subtract 5 seconds to the pace for 200 meters, giving you your respective "on" and "off" splits.
For example, say a 14:00 5k guy typically does his tempo runs at 5:00 mile pace. This pace comes out to 37.5 seconds per 200 meters. So this runner's "on" pace would be about 32 seconds and an "off" pace 43 seconds per 200 meters. For simplicity sake, don't get hung up on decimals, round down for the "on" 200 and up for the "off" 200.
As for the format of the workout, I suggest treating it as you would a cruise/tempo interval session. The repetitions can be of 1000, 1600, 2000, and at the very longest 3200 meters in length. So the workout could be 6-8x1000, 4-6x1600, 3-5x2000, or 2-3x3200 all off 45 to 90 seconds rest depending on the runner's fitness.
The benefits of this workout are:
1. An overall "threshold" stimulus, like a standard tempo run (same heart rate zone)
2. The added turnover stimulus of the fast "on" 200's, at around mile pace
3. Learn the important skill/ teach the body to gear change, in a way that is realistic to races (changing gears while already tired and in the matter of a few steps)
4. Learn to listen to the body and finding the pace
I believe you will find that runners initially find this workout much more difficult than a standard tempo run ON THE LEGS, but AEROBICALLY about the same effort. I mean to say that the constant changing of paces will zap your legs the first few times you try, but the paces are manageable on your lungs and will feel like a tempo run. After a while, you can really tell that your legs and body have adapted and you can change paces with much greater ease, both in the workout and respond in races. You know you are really fit when this session feels comfortable.
Let me know what your thoughts are.