Some runs are best measured by time, others by distance.
If you’ve looked at enough triathlon training plans, you’ve probably noticed that some coaches prescribe run workouts by time, while others do so by distance, and still others go back and forth between the two metrics. And, if you have noticed this pattern of diversity, perhaps you’ve wondered whether you should measure your runs by time or by distance.
The problem coaches face when they design training plans for general use is that individual runners run at different speeds. For this reason, any given run workout, whether based on distance or time, is not really the same workout for every athlete. For example, a two-hour run for a slower athlete who runs 10:00 per mile will come out to be a 12-mile run, whereas a workout of the same duration done by a faster athlete who averages 7:00 per mile will come out to be more than 17 miles.
You can race with equal success whether you plan and execute runs by time or by distance. However, your best bet may be to perform some types of workouts by distance and others by pace.
Measure by Distance
As you know, an iron-distance triathlon has a 26.2-mile run at the end. To build the endurance needed to finish a race of this length, every athlete, whether fast or slow, must complete a few training runs that are relatively close to the marathon distance—at least 17 or 18 miles. Naturally, it takes a slower athlete much longer than it takes a faster runner to run 17 or 18 miles, but then, the same is true for the race itself. When you’re planning and executing run workouts designed to build the endurance you need to go the distance—especially a marathon—it’s better to use distance as your metric.
Measure by Time
Other workouts are intended to develop the physiological attributes needed to race faster. Threshold runs are one example. These consist of a moderately prolonged period of running at anaerobic threshold intensity (or the pace just below which you begin to struggle for breath) sandwiched between a warm-up and a cool-down. Most runners, whether fast or slow, can sustain their threshold pace for about an hour. It’s easy to see why when you consider that 5 miles of threshold-pace running will take 40 minutes for an athlete whose threshold pace is 8:00 per mile, but only 30 minutes for an athlete whose threshold pace is 6:00 per mile. A 5-mile threshold run is therefore a much harder workout for a slower athlete. To get the same level of challenge and benefit, a slower athlete needs to run an equal duration at threshold pace as a faster athlete (30 minutes), but a shorter distance (3.75 miles vs. 5 miles, in this example).