Endurance, on the other hand, is gained much more rapidly. A big factor in endurance—especially Ironman endurance—is the body learning how to burn energy more economically so that race intensity can be sustained longer. This is a primal survival trait that is deeply rooted in our genes. Because of it, if we were put in a situation where our survival was threatened due to a lack of endurance (i.e. food was scarce and we had to travel a long way to hunt), our body would adjust quickly.
This ability to gain endurance rapidly, though, comes with the caveat that we also lose it just as swiftly. This is a major reason why the three-week taper fails for Ironman. We know from experience that we start to see a decline in endurance ability within seven to 10 days.
That’s why the endurance part of training must be maintained until seven to 10 days out from race day.
In the last few weeks before a race you need to structure your training to maximize endurance and maintain strength and speed gains. You don’t need to hammer out long sessions every day to do develop peak endurance; a weekly long ride and long run are enough Nor do you need to perform a large amount of speed and strength training to maintain the speed and strength you developed earlier in the training cycle.