Distance or Time?

A few years ago when I first sat down with my swim coach, who is also a level 1 USAT tri coach, she outlined a plan for me that was based on training by TIME. In other words, figure out how much time your longest race will take you, and then schedule your workouts so that you know you can go hard for that amount of time.

When I sat down this fall with the Triathlete's Training Bible, I had a similar experience...you pick your periodization based on the number of hours in a year you think you'll train (ok, that's drastically simplified) but it was still about TIME, not about the distance covered in that time...

I know when you're training for an Ironman, you might train so that you complete ONE of the THREE distances in one training session but you likely never do the whole ironman distance. But along the same lines, you don't really go out and train for 14 hours...if you think that's how long it's going to take you to finish an ironman.

I know when you train for a marathon or half marathon that you're more likely to focus on milage...distance run during your training sessions. Why would triathlon be any different?

So how do YOU schedule your training? Do you map out your training year by TIME or by DISTANCE? WHY? Anyone feel strongly that there's a right way and a wrong way? Feel free to reference any relevant links too...

Thanks for the imput!