Researchers have found that our minds process distance and time differently. When the finish line of a measured effort is in sight, you get visual feedback about how much farther you have to go, which spurs you to accelerate near the end.
Time feedback, however, is discontinuous: You have to keep glancing at your watch. As a result, studies find that you're more likely to maintain an even pace throughout a time-based interval, but run faster in a distance-based effort. (See what the studies conclude about time vs. distance training.) Both approaches have advantages, depending on the purpose of your run.
Run by Time
...To hone a sense of effort
The late Harry Wilson, coach of former mile world record holder Steve Ovett, had his athletes spend the winter focused on time-based repeats. Runners had to tune into their bodies to identify the pace they could sustain, a vital skill for racing. When running by effort, pay attention to your breathing rate and how your legs feel.
Tempo runs are another workout where getting the feel right is crucial. Once a month, run your tempo by time. Afterward, use your watch or a mapping app to check how far and fast you went.
...And save your psycheRepeating standard measured workouts when you're returning from a break or when you're simply not feeling great can be a blow to the ego, or tempt you to overdo it in order to hit more "respectable" splits. If you know you're not up to your usual standards, hit the roads or trails for a fartlek run. The basic structure of the workout can be the same—just don't measure it.
Run by Distance
...To learn pace
As spring approached, Wilson switched to one track and one fartlek run each week; by summer, all speedwork was on the track. The switch forced runners to focus on their actual goal race pace (and revise it up or down if necessary). The curves and lines of the track provide continuous feedback, enabling you to tap into your finishing instincts—a good dress rehearsal for the real thing.
...And how to kick
Many runners fall into the trap of launching a finishing kick in every track repeat. If you feel fresh enough to sprint at the end of an effort, set out at a quicker pace on the next repetition, rather than ingraining the jog-and-kick pattern. And don't be afraid to finish knowing you had one more gear: "Train, don't strain," as coach Arthur Lydiard used to say.race.