For my first Ironman I did a four-week taper, which made perfect sense to me at the time since back in my swimming days we would do a two week taper for our big swim meets, and when my longest swim race was 16 minutes, wouldn't it make sense to taper even longer for a ten hour event?
That first Ironman was not an abject failure, but my plan to go sub ten didn't quite materialize - 10:39 and a lot of walking on the run. In hindsight, I wasn't truly Ironman-fit for that race, by biggest weeks of cycling and running were in the 300K of cycling and 60K of running range, so there wasn't an Ironman-worthy base to taper down from.
In 2011, I tried the four-week taper again for Ironman Louisville, and faded in the last six miles of the run despite a solid nutrition plan and good pacing. After the race, I looked at my volume for August and realized that I was backing off too much. In an effort to trade endurance for speed, I'd given up too much endurance to get speed that I wasn't going to use in my goal race. Racing an Ironman 'fast' doesn't require you to swim, bike or run all that fast...only about 75-80% of your functional threshold pace, or the pace you could hold for approximately an hour. What is DOES require is the ability to carry that slightly-faster-than-moderate pace for a really, really long time. In that equation, endurance trumps speed.
Going into Hawaii last fall, I decided to taper for one week. This was partly because the race was only six weeks after Louisville, and those two weeks after Louisville involved very minimal training. The decision was also based on my own n=1 empirical evidence than in long-distance triathlon, I tend to race fast when I train through races, and more often that I like, I fall flat at tapered races. The result was strong, so I'm using the same approach for my first race of the 2012 season, Strongman Japan. Training was full-on until yesterday, when I started to back off one week before the race. Today and tomorrow I'll be traveling, then once at the race site I'll do a couple short maintenance workouts to make sure my gear is ready to roll and then we'll let it rip on Sunday.
When tapering for track and cross-country in high school, I used to love the 'crazy legs' feeling I'd get when the rest really took hold of my body, feeling like all I wanted to do was get outside and drill tempo KM's around the block. While I love this feeling, I've come to recognize it's not a good sign for Ironman racing - the 'crazy legs' prior to race day tend to indicate there's been too much rest.
What I'm hoping for is I've found the right balance between training stress and rest so that I get that 'crazy legs' feeling at mile 20 of the marathon on Sunday so I can float on home to the finish in the Miyako-jima Stadium, just like Sam & Dave killing it in Europe back in '66: