Triathletes usually do intervals of uniform length in their interval running workouts (8 x 400 meters, 4 x 1K, etc.). I know I do. Just this morning I ran 10 times one minute (or 360 yards) uphill.
It's quite sensible to perform intervals of uniform length within a single workout. The goal of most such workouts is to challenge your capacity to work at a particular intensity level. Intervals of different lengths are best suited to challenging your capacity to run at different intensity levels.
Naturally, shorter intervals are better suited to higher-intensity efforts, while longer intervals are better suited to lower-intensity efforts. It's most practical and effective to run only intervals of the length that is best suited to the specific intensity level you're targeting in a given workout.
However, there are times when it's a good idea to target more than one intensity level in an interval running workout. In these instances, you'll want to include intervals of varying lengths, each appropriate to the running intensity targeted in that single effort. This type of workout is known as a mixed-intervals workout.
Timing of Mixed-interval Workouts
When are mixed-interval workouts appropriate? I like to schedule them for the peak phase (the final four to eight weeks) of training. In this phase, the intensity level you should target is race intensity.
But, it's important to also do some interval work at other, primarily faster, pace levels to help you sustain fitness adaptations earned by doing larger amounts of work at these levels earlier in the training cycle.
You certainly don't want to burden your body with a lot of blazing-fast running in the final weeks before a peak race. Triathletes who phase out very high-intensity training altogether in the peak phase tend to lose their highest gears and become two-speed athletes (endurance pace and race pace) who quickly fall apart whenever they exceed their average goal race pace even slightly. That's a limitation you want to avoid.
I recommend that most triathletes do two higher-intensity running workouts per week in the peak phase of training. One of them should be a tempo workout performed at or near your goal race pace. The other can be a mixed-intervals workout that includes efforts at race pace plus efforts at other, primarily faster, pace levels.
Such workouts will provide an additional stimulus to boost your efficiency and fatigue resistance at race pace as well as just enough work at primarily higher intensity levels to maintain the increases in fast-twitch muscle-fiber recruitment capacity and VO2 max that you earned earlier in training.
I recommend a descending format for mixed-interval workouts, with the slowest intervals first and the fastest intervals last. If the order is reversed you risk training yourself to slow down as you fatigue.
Your first and longest interval should be run at--or a little slower than--race pace. Hence, the exact pace of this opening interval depends on the length of the race you're training for. The last interval should be run at 5K running race pace or faster, and I encourage you to sprint the last 100 yards of this final interval.
In doing so you will activate motor units in your running muscles that are never used at a slower pace. Active recovery (i.e. jogging) periods between intervals should be just long enough to allow you to perform the next interval at your highest level.
The following are suggested mixed-interval workout formats for each of four race distances: sprint, Olympic, half-Ironman and Ironman. There's a beginner/intermediate and an intermediate/advanced version of each.
Be sure to warm up with at least five minutes of easy jogging (and ideally some dynamic stretching) and cool down with at least five more minutes of jogging.