The original post dealt with their diet and that is what I was referring to. Before I responded, I did look at the past diets of some tri-atheletes that I have worked with and their training. Most of them do a cross-training of sprints, distance, and intervals. Not everyone follows the same diet, but from what I have gathered over the years, the diet that I stated is what most of them have been doing. Be it right or wrong, everyone has stated that they carb-load, so they intake a lot of complex carbs and try to keep their fats healthy, but low. Protein is moderately high with very clean sources.
For the training aspect, staying in your target heart range, even while sprinting, can be possible as long as you keep you resting periods extremely short and make your workout aerobic in nature by coupling multiple exercises together in a sequence. It has more to do with the intensity rather than the duration. Remember, this information was gathered from different atheletes over the years, so it is not a "law" to follow, just a guideline.
If you know of something different, post it for rjx, so he gets the whole picture.