On race day - how much faster on the bike?

JD, the thing that makes the top performers the top is their ability to burn fat at a higher intensity. Now a lot of that may be genetic but it is one of those things you can train.

People do short bricks when they are well fuelled and invariably do them at the wrong intensity because they can. Then crash and burn on race day after a good first 10k or just suffer from the run start.

If you are depleted of most of your carbs and only using what you are eating and your fat stores to keep moving, then you are in the same position as you would be getting off the bike in an IM. Your body adapts to specific stresses and if you do not simulate them in training there is no way it can adapt and come race day it will react negatively. This also goes for the way it feels to run long after a long bike, it is just not the same as a short run and having experience of that makes it easier to deal with.

There is another benefit of exercising in a depleted way and it is that it stimulates mitochndrial production which as we all know are the powerhouses behind any aerobic performance. The science is still a bit grey on the mechanism but it is a proven measureable result, confirming something pro cyclists have known for decades but which sport scientists have always poo poo'd up till now.

Exactly, that is why it is not all about the bike  

I think this needs a new thread

JD, the old cyclist way was to do a long ride, not fuel much after and then go out the next morning with no carbs at all, not even a sugar in your tea or coffee and ride hard till you blew up.

I read about a more scientific approach recently which said

* Have a low-carbohydrate meal prior to the session and then beginning the workout with 45-60 minutes of low-intensity, steady cycling at around 70 percent of your VO2 max (a level where you can still breathe through your nose, with your mouth shut).

* After your glycogen is sufficiently depleted, switch to intervals of five minutes' hard exercise with a minute of rest in between.

I think to be honest the best way is to avoid blowing up (I just like that feeling ) and try to keep on the limit without going over.