Tempo Run vs Intervals: What Each One Actually Trains

What does each one actually train?

Intervals mainly raise your aerobic ceiling, your VO2 max, largely by making the heart pump more blood per beat. Tempo runs mainly raise how much of that ceiling you can use for a long time, the speed you can hold before lactate builds. One lifts the roof. The other lets you live closer to it.

The clearest head-to-head evidence is a 2007 study by Helgerud and colleagues in Medicine and Science in Sports and Exercise. Forty moderately trained men trained three times a week for eight weeks in one of four ways, all matched for total work: long slow running at 70% of maximum heart rate, threshold running at 85%, short 15 second intervals, or four repeats of four minutes at 90 to 95% with three minutes of easy running between them. The four by four minute group raised VO2 max by 7.2%, from 55.5 to 60.4 mL/kg/min, and both interval groups beat the threshold and slow groups significantly. Stroke volume rose by about 10% after interval training. So if the goal is a higher ceiling, intervals do that job better than tempo runs.

That does not make tempo runs second best. The study measured the ceiling, and a race is not run at the ceiling. It is run at the fraction of it you can sustain, which is exactly what threshold work targets. Getting into the physiology of threshold would take a post of its own, so here we stay practical.

Which matters more for a 5K, a 10K, a half or a marathon?

The longer the race, the more it leans on threshold. A 5K is run well above threshold, close to your ceiling, so intervals are the most race-specific work. A 10K sits in between and uses both. A half marathon is run close to threshold, and a marathon slightly below it, which makes tempo work the bigger lever for those two.

The classic evidence here is a 1979 study by Farrell, Costill and colleagues on 18 male distance runners. The running speed at which lactate began to accumulate in their blood tracked race performance at every distance tested, from 3.2 km to the marathon, with correlations of 0.91 or higher, stronger than VO2 max at 0.83 or higher. Its marathoners raced only a few meters per minute faster than that speed. The longer your race, the more your result depends on the pace you can sustain rather than the height of your ceiling, which is also why a high ceiling does not guarantee a fast race.

One honest caveat. A 2007 review by Midgley, McNaughton and Jones concluded that there are too few well controlled training studies in trained runners to give firm recommendations for improving threshold or VO2 max. The mix above is a sound reading of the physiology, not a proven formula.

How many hard sessions a week should you do?

For most recreational runners, one or two. If you run three or four times a week, one hard session is enough, alternating tempo one week and intervals the next. If you run five or more times, a tempo and an interval session with at least two easy days between them is a common and sensible shape.

The rest of the week does the quieter work. Easy running builds the base that hard sessions stand on, which we covered in why easy days make you faster. The real test of how much of your week should be hard is whether you arrive at each session fresh enough to hit it properly.

How can you tell from your watch that a session hit its target?

Judge intervals mostly by pace and effort, because heart rate lags behind them. Judge tempo runs by heart rate and pace together, because over 20 minutes or more heart rate settles and becomes the more honest signal. In both cases, look at the shape of the heart rate line, not just the average.

In a short repeat, heart rate can take a minute or more to catch up with the effort, so on 30 second or one minute repeats it may peak only in the last few, or never reach zone 5 at all. In short intervals, a heart rate that never reaches your target zone does not mean you ran too soft. Check that your pace held steady across repeats. For four minute repeats, heart rate should climb into the 90% range by the second half of each one and step a little higher from the first repeat to the last.

In a tempo run, watch for cardiac drift: heart rate creeping upward at the same pace as the run goes on, which heat, dehydration and fatigue all make worse. A few beats over 20 minutes is normal. If your heart rate climbs steadily into zone 4 at an unchanged pace, the session has quietly become harder than a tempo, and slowing down keeps it doing its job. The same run on a hot day or after bad sleep simply costs your heart more than it did last week.

FitWoody is an iOS app for iPhone, iPad and Apple Watch that turns your watch data into a clear read on how your body is doing. For hard sessions, the useful parts are custom heart rate zones set from your own numbers, cardiac load that scores each run by what it cost you internally, training load tracked against your own baseline, and recovery baselines built from HRV, resting heart rate and sleep. Flyover replays a run along its route, with heart rate moving as you went. An Apple Watch is required, and it informs rather than diagnoses.

Pick the session your race needs most, do it once this week, and keep the rest easy. Showing up, in any form, counts.