Why pace zones matter more than arbitrary effort
Your body adapts to the specific stress you apply. Easy running builds mitochondrial density and fat oxidation — adaptations that only happen at low intensity. Threshold running raises your lactate threshold — only possible when sustained precisely above the point where lactate starts to accumulate. VO₂max intervals develop maximum aerobic capacity — only achieved at near-maximum effort.
Running all sessions at medium effort means you're too fast to produce the easy-running adaptations, and too slow to reach the threshold or VO₂max adaptations. You get the fatigue without the fitness.
Pace zones solve this by defining exact effort ranges calibrated to your current fitness. Once your zones are set, the question 'how hard should I run today?' has a specific, non-arbitrary answer.
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Easy pace: the most under-used zone
Easy pace is the foundation of all endurance training. It's the effort at which mitochondrial density increases, fat oxidation improves, and your aerobic base becomes capable of supporting harder sessions. Most runners run 70–80% of their total volume in this zone.
Easy pace is typically 1.13–1.25 times your goal half marathon pace per kilometre. For a runner targeting 2:00 (5:41/km), easy pace is 6:26–7:06/km (10:22–11:25/mi). Most runners at this level are running 6:00–6:15/km on what they call 'easy runs' — which is too fast.
The test for easy pace is simple: you should be able to speak in complete sentences without pausing for breath. If you can't, you're not in the easy zone. GPS watches frequently display paces that feel insultingly slow — that's the right signal.
- →Purpose: builds aerobic base, fat oxidation, mitochondrial density
- →Effort: fully conversational — full sentences without gasping
- →Heart rate: zone 2, 65–75% of maximum heart rate
- →When: all recovery runs, most long runs, the majority of weekly mileage
Tempo pace: raising your lactate threshold
Tempo pace — also called threshold pace or T-pace in Jack Daniels' VDOT system — is the effort you could sustain for approximately 60 minutes at peak fitness. For most half marathon runners, it sits 0.89–0.93 times their goal HM pace per kilometre. For a 2:00 half marathon runner (5:41/km goal), tempo pace is 5:03–5:17/km (8:08–8:30/mi).
At tempo effort, you can speak 2–3 words but not full sentences. It's 'comfortably hard' — a phrase that only makes sense once you've run a proper threshold session. Too many runners describe this zone as 'moderate-hard', running at 90% of effort when they should be at 80%. The result: sessions that are fatiguing but not adaptive.
Tempo sessions are typically 20–40 minutes of sustained threshold effort, preceded and followed by easy warm-up and cool-down running. The duration is what distinguishes a threshold session from a race — you're training at this effort, not competing at it.
Speed/interval pace: developing VO₂max
Speed sessions — called I-pace (interval pace) in Daniels' system — target your VO₂max, the maximum rate at which your body can consume oxygen. This is the ceiling on aerobic performance. Intervals at this intensity are run at 0.83–0.87 times your goal HM pace per kilometre, typically in repetitions of 400m–1200m with rest periods between.
For a 2:00 half marathon runner (5:41/km goal), speed pace is 4:43–4:57/km (7:36–7:59/mi). At this effort, conversation is impossible — it's 8–9 out of 10 effort, sustainable for 3–5 minutes per repetition.
Speed sessions are the most individually punishing sessions in a plan, which is why most plans include only one per week and only in the speed-specific phase (typically weeks 7–9 of 12). Doing too much speed work too early produces overreaching — fatigue without adaptation.
- →Purpose: raises VO₂max ceiling, improves running economy at speed
- →Effort: 8–9/10 — unable to speak, controlled rather than desperate
- →Heart rate: zone 5, 88–100% of maximum heart rate
- →Format: 400m–1200m repetitions with full recovery between reps
Long run pace: why it should be slower than you think
The long run's purpose is time on feet at aerobic effort — not race-pace simulation, not a test of fitness. Long runs should be run at 1.20–1.25 times goal HM pace per kilometre. For a 2:00 runner, that's 6:49–7:06/km (10:58–11:25/mi) — significantly slower than goal race pace.
Running the long run too fast is the single most common training mistake in half marathon preparation. It doesn't produce additional fitness benefit over easy long runs, and it significantly increases recovery time — often bleeding into the following week's quality sessions.
The long run becomes truly productive when you finish feeling like you could have continued for another 3–4 km. If you finish exhausted, you ran too fast or too far.