Interval Builder

Interval Progression Strategies

Building interval workouts is only the first step — progressing them over weeks and months is what drives fitness gains. There are four main progression levers: increase the number of repetitions (e.g., 4×800m → 5×800m → 6×800m), increase the interval duration while maintaining pace, decrease rest periods between intervals, or increase the target pace. Only change one variable at a time — adjusting two simultaneously risks overtraining. A proven 4-week cycle: Week 1 introduces the workout at moderate effort, Week 2 adds one repetition, Week 3 reduces rest by 15–30 seconds, and Week 4 is a deload with fewer reps at easy pace. For VO2max intervals (3–5 minutes at 95–100% max HR), research suggests 2–3 sessions per week during a focused block of 4–6 weeks yields the greatest improvement. Threshold intervals (10–20 minutes at lactate threshold) respond well to volume increases — start with 2×10 minutes and build toward 3×15 minutes over 6 weeks. Always include a proper warm-up of 10–15 minutes before intervals, and a cool-down of at least 5 minutes afterward. The workout builder lets you export these structured sessions directly to Garmin devices for guided execution.

How to Import Workouts into Garmin Connect

Garmin Connect allows importing workouts via JSON files. After downloading your workout file:

1. Open Garmin Connect on the web (connect.garmin.com)
2. Go to Training → Workouts
3. Click Import Workout and select your downloaded .json file
4. The workout appears in your library and syncs to your watch on next connection
5. On your watch: Training → Workouts → My Workouts

Types of Running Intervals

Jack Daniels categorises running intervals by intensity relative to VO₂max:

• Interval (I-pace): ~95–100% VO₂max. Classic 1000m–1200m reps. Develops VO₂max directly.
• Threshold (T-pace): ~84–88% VO₂max. Tempo runs of 20–40 min or cruise intervals. Raises lactate threshold.
• Repetition (R-pace): ~105%+ VO₂max. Short 200–400m reps with full recovery. Develops speed and economy.

Why Interval Training Works

Interval training — alternating hard effort bouts with recovery periods — is one of the most time-efficient ways to improve endurance fitness. The physiological benefits come from repeatedly stressing your aerobic system above the intensity you could sustain continuously. During a hard interval your heart rate climbs, cardiac output increases, and your working muscles demand more oxygen than easy running requires. That stress, applied consistently over weeks, triggers adaptations: your heart pumps more blood per beat (increased stroke volume), your muscles develop more mitochondria, and your body becomes better at clearing lactate.

Three adaptations make intervals particularly valuable for runners and cyclists:

• VO₂max improvement. Training at or near your maximal aerobic capacity — roughly 95–100 % of max heart rate — challenges the ceiling of your oxygen uptake. Short intervals of 2–5 minutes at this intensity are the most effective stimulus for raising VO₂max.
• Lactate threshold gains. Cruise intervals and tempo efforts at 84–88 % of max HR push the pace at which lactate accumulates faster than your body can clear it. Over time that threshold shifts upward, letting you run faster before fatigue sets in.
• Running economy. Short, fast repetitions (200–400 m at mile pace or faster) develop neuromuscular coordination and stride efficiency. A more economical runner uses less oxygen at any given pace — which matters enormously in a race.

Anatomy of an Interval Session

Every well-structured interval session has four phases. Skipping any of them increases injury risk and reduces the training benefit.

1. Warm-up (10–20 min). Easy aerobic running or cycling to raise core temperature, increase blood flow to working muscles, and prime your neuromuscular system. At least 10 minutes; 15–20 minutes before a hard VO₂max session.
2. Work repetitions. The hard effort bouts — the heart of the session. Duration, intensity, and number of reps depend on the session goal.
3. Recovery intervals. Active recovery (easy jogging or very light pedalling) between work reps. Active recovery clears lactate faster than standing still and keeps your cardiovascular system primed for the next rep.
4. Cool-down (5–15 min). Easy aerobic effort to gradually lower heart rate, flush waste products from muscles, and begin the repair process. At least 5 minutes; longer after a high-volume session.

The Interval Builder structures your session exactly this way — set your warm-up and cool-down duration at the top, then add one or more repeat blocks in the middle.

Common Session Types and Work:Rest Ratios

Choosing the right session type depends on the fitness quality you want to develop and where you are in your training cycle. Here are the four most useful types with typical structures:

VO₂max Intervals (3–5 min work / 2–3 min recovery)

Target intensity: 95–100 % of max heart rate, or your fastest pace you can hold for about six minutes (roughly 3000 m race pace for runners). Classic formats include 5 × 3 min with 3 min easy jog recovery, or 4 × 1000 m with 90 s recovery. The work:rest ratio is typically 1:1 or up to 1:0.75. These sessions are demanding; two per week is typically the upper limit during a focused block.

Threshold / Cruise Intervals (8–20 min work / 1–3 min recovery)

Target intensity: 84–88 % max HR, or a “comfortably hard” effort you could sustain for about an hour — your lactate threshold pace. Cruise intervals break a long threshold run into chunks with short recoveries: 3 × 10 min at threshold / 90 s easy is a classic beginner format. Work:rest is roughly 5:1 to 10:1.

Short Speed Reps (30–90 s work / 2–5 min full recovery)

Target intensity: 105–115 % VO₂max, or mile race pace and faster. Classic format: 8–12 × 200 m with 2–3 min full recovery. Full recovery (work:rest of 1:4 or more) is essential; the reps should feel controlled and fast, not laboured.

Hill Repeats (30–90 s hard uphill / easy jog-down recovery)

Hill repeats develop leg strength and running economy with lower impact stress than flat speed work. A 6–8 % gradient at hard effort for 60–90 s with a slow jog-back recovery combines strength and aerobic stimulus. Build to 6–10 reps over several weeks.

Pace vs HR vs Power: Which Target to Use

All three metrics can guide interval intensity, but they each have practical trade-offs.

• Pace is the most immediate feedback for runners on flat terrain. It responds instantly — you can adjust within seconds. Ideal for short intervals (under 3 minutes) where heart rate hasn’t had time to stabilise.
• Heart rate lags by 30–90 seconds on short reps — your HR at the end of a 60-second interval may not reflect true effort during it. HR is better suited to threshold and longer aerobic intervals (5+ minutes) where it has time to reach a steady state.
• Power (cycling or running power meter) responds instantly like pace but is terrain-independent. On a hilly course or into a headwind, power keeps effort honest when pace becomes misleading.

For most runners without a power meter: use pace for short reps (under 3 min) and HR as a secondary check; use HR as the primary guide for threshold intervals and longer work.

Worked Example: A VO₂max Session for a Club Runner

Here is a complete session for a runner targeting a sub-45-minute 10 K, with 5:30–5:40/km threshold pace and roughly 3:50–4:00/km VO₂max pace:

• Warm-up: 15 min easy (6:30–7:00/km), including 4 × 20 s strides at the end.
• Main set: 5 × 1000 m at 3:55–4:05/km (VO₂max pace) with 90 s easy jog recovery.
• Cool-down: 10 min easy (6:30+/km).

In the Interval Builder, create one repeat block: Reps = 5, Work = 1000 m distance, Recovery = 90 s, Pace target fast end = 3:55, slow end = 4:05. Add a 15-minute lap-button warm-up and 10-minute cooldown, then download the Garmin JSON and import it to Garmin Connect.

Common Mistakes to Avoid

• Running intervals too fast. The most common error. Starting rep one at 110 % effort means rep five falls apart. The correct feel for VO₂max intervals is hard but controlled — you should be able to hold pace across all reps.
• Too little recovery. Cutting rest short to save time forces later reps at lower intensity, defeating the purpose. Honour the prescribed recovery before the next hard effort.
• Too much interval training. One or two quality sessions per week is sufficient for most athletes. Easy runs should make up 75–80 % of total weekly volume.
• Skipping the warm-up. Going hard on cold legs sharply increases the risk of hamstring and Achilles injuries. The warm-up is not optional.
• Changing too many variables at once. Increase reps OR pace OR decrease rest between sessions — never all three simultaneously.

Frequently Asked Questions

How many interval sessions should I do per week?

For most recreational runners and cyclists, one to two structured interval sessions per week is optimal. The remaining sessions should be easy or moderate aerobic work. More than two hard sessions weekly requires careful periodisation and adequate recovery.

Can I do interval training on a treadmill or stationary bike?

Yes. Treadmill intervals are excellent for precise pace control and are lower-impact than road running. Set the incline to 1 % to approximate outdoor effort. On a stationary bike, use power (watts) rather than speed as your target metric for consistent effort.

How long before a race should I stop doing hard intervals?

A general rule of thumb: complete your last hard VO₂max session 10–14 days before an A-race. Shorter, sharper reps (strides, 200 m reps at race pace) can continue up to 4–5 days before.

What is the difference between interval training and fartlek?

Fartlek (Swedish for “speed play”) is unstructured — you vary effort based on feel, terrain, or landmarks, without fixed durations or targets. Interval training is structured: fixed work durations, fixed recovery durations, and measurable intensity targets. Both have a place in a balanced training plan.