My 2 week old Z-Girl is unsure of the answer to this, in fact I think she is only sure of two things in this world currently; sleep & breast milk!
One of the most common debates in run training is whether it is better to base workouts on time or distance. In the world of ultra races, running based on time is an accepted approach.
A lot of the IM workout schedules do base a lot of the training on getting the 'hours' in rather than the distance. Working on the endurance and mental fatigue of the the athlete first and foremost.
Straight out of some resent research shows:
" First, the researchers had all 38 kids run a 750-meter time trial as fast as they could. Then they split them into two groups—one ran the 750-meter trial again and the other ran for the amount of time it took them to run the first trial. For the former group, they managed to cover the 750 meters in about the same amount of time. The other group who ran based on time, however, covered significantly less distance than they did the first time. The researchers concluded that this was likely a result of the fact that time cues are less tangible and therefore make it more challenging to judge pace than information based on distance where, for example, you can actually see the finish line. Give yourself a concrete distance to complete for your next all-out workout or time trial and you might just go a little faster. "