Interval Workouts

What intervals do

An interval workout is a set of hard reps with easy jogging between them. The stimulus is time spent near your top aerobic effort, where your body is using close to the most oxygen it can (your VO2 max). The recoveries let you collect more of that time than one continuous hard effort would. Over weeks of this, your top end rises and fast running costs you less. See the guide on VO2 max.

Intervals sit on top of easy running and do not replace it. Keep your easy volume and use intervals for the top end and for efficiency.

Where they fit in your plan

Build a base of easy running first, then threshold work, and add intervals on top. A fast change in volume or intensity is what injures amateurs. An interval session is a hard session: count it in your two a week, and never put it next to another hard workout. Once a week of interval work is enough for most runners.

The place of intervals changes with the race:

• 5K and 10K: VO2 max work, hill repeats and hill sprints help you express the fitness you have more than they build it, so cluster them in the last 6 to 8 weeks. Before that, about two aerobic sessions to one faster session. See the guide on 5K and 10K training.
• Half marathon: speed work at 5K pace, with little VO2 max work until the last quarter of the block. See the guide on half marathon training.
• Marathon: a sharpening tool and not the main event. In the race-specific weeks speed work stays at 10K pace, for example 800s with short rest. See the guide on race-specific preparation.
• New to speed: start with strides, and add an occasional short interval session only after three to four weeks of comfortable strides. A newer runner may not be durable enough yet for really hard days. See the guide on strides.

How hard and how much

Run the reps hard and controlled, at roughly 10K down to 5K effort, not all out. A faster session should finish at 6 or 7 out of 10 effort, not 9 or 10. In 19 male runners over 6 weeks of interval sessions, 5K time improved 3.0% when the fast part of each set was run at a submaximal effort and 2.3% when it was run flat out, and VO2 max rose about 7% in both groups¹.

Most reps run about three minutes or less. Past three minutes, run them relaxed unless you are training to hold a specific pace, because faster is not better once the reps get long. Look for 10 to 20 minutes of hard running in total, with reps of 1 to 4 minutes or 30 seconds hard and 30 easy, and 5-minute reps only for the strongest runners. A newer runner sits at the low end with short, bite-sized sessions, and more volume goes inside the session with experience:

• Newer runner: 6 x 2 minutes or 6 x 3 minutes hard, with easy jogging between.
• Experienced runner: 5 x 3 minutes hard with 3 minutes easy.
• Very strong runner: 4 x 5 minutes hard with 2.5 minutes easy.
• Short reps: 200s at 5K down to 3K pace, or 300s and 400s at 10K down to 5K pace, with a jog between. A fartlek of 1 minute on and 1 minute off works the same way. Short reps let you correct your pacing mid-session, which long repeats do not, so start here.

Recovery and progression

Recover with easy jogging, about as long as the rep for 3-minute reps and about half as long for 5-minute reps. Take enough that the next rep holds the same quality. Warm up with easy running and a few strides before you start.

When a session gets easy, change one thing: more reps, longer reps or shorter recovery. A session is a stimulus you adapt to, and then it has to change.

If a session goes badly, ask what it is for. If the point is time at the effort, the exact pace matters less, and you can slow down a little and finish. If the point is a specific pace and you are well short of it, stop, or break the work into shorter reps, for example 5 x 400 with a jog in place of mile repeats. If you are not ready for the hard session at all, run an easy long run or an aerobic tempo instead (see the guide on tempo and threshold runs) and bring the intensity back in a couple of days.