This is when the doubts start to creep in. This is when you start doing things you shouldn't do during a taper. You start pushing harder to make sure you eek out every bit of speed for the race, or you do that "one last" track workout or long ride "just to be sure" your fitness is OK. Problem is, in the final weeks those workouts are the ones that can cripple your ultimate performance.
If the work hasn't been done before tapering begins, then it's too late. If you're still under trained - due to recent sickness, injury, laziness, etc, - still, the only way you're going to get your body totally prepared is by tapering and not by working harder.
I suggest a 2-week taper for Olympic distance triathlons and 40k TT's on the bike, or 10k runs. For marathons, Ironmans and ultra-cycling events, I give my athletes a month-long taper. Weekly volume tapers off dramatically, and the intensity of the hard workouts stays high though the volume of hard work done also decreases. During this decrease in overall work the body takes less time to recover from workouts and, thus, is able to top off its energy systems stores - something it can't usually do during hard bouts of traning.
Finally, don't misinterpret the "mid-taper blues", a period of time where you can feel sluggish. Its very common to the taper process. Your body has been used to getting hammered and all of a sudden you're being nice to it. It takes a while fo the the body to adapt and round the corner. If you encounter the blues, don't panic. You'll rebound quickly. When you do, you should be feeling more energetic stronger, faster and confident.
Cutting down on duration, amount of intensity and, possibly, the number of weekly workouts is the way to maximize the taper process. Doing that last hard/long workout just to be sure your fitness is OK, is not the way to ensure great results.
Now, some coaches will state the opposite. Their premise is that the more intensely you train (i.e., the higher the HR you sustain during your training), the more time you need to recover so you can peak for an event. False, I say.
Intense training makes you fast. Yes, added rest will allow you to be more "sharp" and, thus quicker. But if you rest too much, you'll lose fitness and be flat; or, you'll be sharp but unable to sustain your newfound speed because you've slacked off the most important aspect of your training -- endurance. Make no mistake, even at the Olympic Distance, triathletes are endurance athletes not speed freaks.
Before you think that a month-long taper for a 70.3 or 140.6 is crazy, hear me out. What's crazy to me is "Triathlete" magazine publishing an article from a well-known coach on "13 weeks to a respectable Ironman", where in this article the coach suggests an athlete complete his/her longest run only 2 weeks before the main event, in excess of 3 hours. In my mind, this approach will not lead to the best performance an athlete can demonstrate.
Long hours of training, in any discipline, pounds the body. You may be more sore or tired immediately following a hard interval workout, but the repercussions of a long-distance workout last far beyond the effects of an interval workout. For example, the rule of thumb for recovering from a hard run is "Avoid hard running for the same number of days as miles you ran in the race". So, after a 5k, you would wait 3 days before running hard again; for a marathon, it would be close to a month.
Thus, I have my ultra athletes complete their longest week of training 4 weeks out from their main event. From there, each week (and each long workout) drops 20-25% until the final 2 weeks are pretty minimal in duration. To counteract the drop in volume, the athletes complete intervals at a slightly higher rate of exertion than is typical for their training.
However, this is counterbalanced with a higher work/rest ratio, which leaves the athletes fresher for each repeat, as well as fresher at the end of the workout.
The best thing you can do for yourself in the final weeks leading up to a major competition is to rest and relax. This doesn't mean "kick back in a chair" but rather significantly cut your volume and slightly increase the intensity of your hard workouts (as they get shorter as well). If you've put the work in and have been honest with your efforts, you'll hit the starting line ready to turn in your best performance to date.