While half marathons and marathons are matters of resisting fatigue during the later miles, 5K and 10K racing is like fighting off a lactic acid firestorm for the final third of the race.
No matter how comfortable the early pace may feel to you, about two thirds of the way into a fast 5K or 10K a spark suddenly catches fire and starts to spread rapidly as your legs begin to lock up and your stride shortens ever so slightly. Your quads are screaming at you to stop and your upper body tenses up as you seemingly start going backward while you struggle to maintain pace or stick with the runner in front of you.
There's nothing wrong with any of this at the end of a hard race, of course; it simply means that you're doing it right. While your muscles are inevitably going to catch fire toward the end of a competitive 5K or 10K effort, you can train your body to slow down the burn and better handle the demands of the race in training. One of my favorite ways to do this is with the descending ladder workout.
Begin by warming up with 15 to 20 minutes of easy jogging followed by six 20-second strides (faster accelerations) to get your fast-twitch muscles firing. After your warmup is complete, run for 10 minutes at your current half-marathon race pace. If you've never raced a half marathon and are unsure of how fast to run this part of the workout, add about 15 seconds per mile to your 10K race pace or 30 seconds per mile to your 5K pace. The McMillan Calculator is also a handy tool that can help get you in the ballpark. The goal here is to inject some fatigue into your legs without totally wiping you out just yet.
After running 10 minutes at half-marathon pace, jog slowly for five minutes as recovery. Once those five minutes are up, begin a descending ladder of pickups, starting with six minutes at your goal 10K pace -- no faster. Upon completion of the six-minute pickup, jog for three minutes as recovery. Continue to step down the ladder with faster pickups of five minutes at 10K race pace, four minutes at 5K race pace, three minutes at 5K race pace, two minutes at 3K race pace (roughly 15 seconds per mile faster than your 5K pace) and, finally, one minute at 3K race pace. The recovery between each pickup is an easy jog for half the duration of the preceding interval, so 2:30 after the five-minute pickup and so forth.
Another way to perform this workout is by doing specific intervals on the track. Perform the same warmup described above, followed by 10 minutes of running at your half-marathon race pace (preferably off the track). After jogging easily for five minutes to recover from that effort, step onto the oval and begin with 2,000 meters (five laps) at your 10K race pace. Jog easily for half the duration it took you to complete that opening interval on the track and then run four laps at the same pace. Continue stepping down the lap ladder (three laps, two laps, one lap), increasing the intensity with each repetition to the point where you're running faster than your goal 5K race pace for the final two intervals. The recovery between each faster interval should be half the time of one you just completed.
This session is demanding, but then again 5K/10K racing is too, so if you want to simulate the burning your legs are going to feel in the final mile or two of a race, try stepping down the ladder in your next workout.