Training

Ultra-running is fundamentally an aerobic event. Even in a fast 50K, more than 95% of the energy demand is met by aerobic metabolism — predominantly fat oxidation at sub-threshold intensities. In a 100-mile race lasting 20-30 hours, the aerobic contribution approaches 99%. Yet many ultra-runners spend a disproportionate amount of training time at moderate intensity (zone 3) that is too hard to develop the aerobic base and too easy to improve top-end speed. The result is a "middle-ground" aerobic system that fatigues prematurely when races stretch beyond 6-8 hours.

Zone 2 training — defined as 60-70% of maximum heart rate or 55-75% of heart rate reserve — is the specific stimulus that builds the ultra-runner's aerobic engine. Research from Inigo San Millan's lab at the University of Colorado demonstrates that consistent zone 2 work increases mitochondrial density by 20-40%, improves fat oxidation rates by up to 30%, and enhances lactate clearance capacity — all adaptations that directly determine how long you can sustain effort before fatigue sets in. The evidence is clear: 80% or more of your weekly volume should be in zone 2.

Why Zone 2 Matters More for Ultra-Runners

Zone 2 trains the aerobic system at the intensity where the largest volume of fat is oxidised for fuel. For ultra-runners, this is the critical adaptation because glycogen stores — roughly 2,000 kcal for a trained athlete — are exhausted within 90-120 minutes at moderate intensity. A 100-mile race requires 8,000-12,000 kcal of energy. The only way to cover the gap is through fat oxidation, which provides a virtually unlimited fuel source (even a lean athlete carries 30,000-50,000 kcal of stored fat).

• Mitochondrial biogenesis: Zone 2 stimulates PGC-1alpha, the master regulator of mitochondrial production. More mitochondria mean more cellular power plants available to oxidise fat and produce ATP aerobically
• Fat transporter upregulation: Consistent zone 2 training increases the density of FAT/CD36 transporters on muscle cell membranes by 15-25%, allowing more fatty acids to enter cells for oxidation
• Capillary density: Zone 2 volume drives capillarisation — the growth of new capillaries around muscle fibres. More capillaries mean better oxygen delivery and waste removal, directly improving endurance
• Lactate shuttle efficiency: Zone 2 training improves MCT1 transporter function, allowing muscles to reabsorb and use lactate as fuel rather than accumulating it as fatigue-causing waste

Heart Rate Targets for Zone 2

Precise heart rate targeting is essential because zone 2 is a narrow band. Too high and you drift into zone 3, losing the fat-oxidation stimulus. Too low and the training stimulus is insufficient to drive mitochondrial adaptation:

• By percentage of max HR: 60-70% of maximum heart rate. For an athlete with a max HR of 185 bpm, zone 2 is 111-130 bpm
• By heart rate reserve (Karvonen method): 55-75% of HRR. This method accounts for resting heart rate and is more accurate for trained athletes with low resting HR
• By feel: You should be able to hold a conversation in complete sentences. If you can only speak in short phrases, you are above zone 2. If you can sing, you are below it
• By lactate: Blood lactate between 1.5 and 2.0 mmol/L — the intensity at which lactate production and clearance are balanced. This is the gold standard for zone 2 identification

On hilly terrain, maintain heart rate targets by adjusting pace aggressively. Walking uphills to keep heart rate in zone 2 is not a sign of weakness — it is precise training execution. Many elite ultra-runners walk every climb in training for this reason.

Weekly Volume Recommendations

Ultra-running performance correlates strongly with weekly training volume — more than with any other single variable. The minimum effective volume for ultra-marathon preparation depends on target distance:

• 50K: 60-80 km per week, with a long run of 25-32 km. Zone 2 volume: 48-64 km (80% of total)
• 100K: 80-120 km per week, with a long run of 30-42 km. Zone 2 volume: 64-96 km
• 100 miles: 100-150 km per week, with a long run of 40-56 km or back-to-back long runs of 32 km + 25 km. Zone 2 volume: 80-120 km

Build volume gradually — increase weekly distance by no more than 10% per week and include a recovery week (reduce volume by 30-40%) every fourth week. The remaining 20% of weekly volume should include one quality session: either a tempo run at lactate threshold or hill repeats for muscular endurance.

The 80/20 Polarisation Model

The polarised training model — 80% easy (zone 1-2), 20% hard (zone 4-5), and minimal time in zone 3 — is supported by decades of research across endurance sports. A 2014 meta-analysis by Stoggl and Sperlich found that polarised training produced superior VO2max improvements, time-trial performance, and body composition changes compared to threshold-focused or high-volume training models:

• Zone 1-2 (80% of volume): Long runs, easy runs, recovery runs — all at conversational pace
• Zone 4-5 (15-20% of volume): Tempo intervals, hill repeats, race-pace efforts — 1-2 sessions per week maximum
• Zone 3 (less than 5%): Actively avoid this zone. It is too intense to accumulate volume without excessive fatigue and too easy to drive top-end adaptation. Zone 3 is where runners get stuck in the "moderate trap"

For ultra-runners, zone 3 avoidance is especially important because the fatigue cost is high relative to the adaptation gained. Every minute spent in zone 3 during training requires 2-3x the recovery time of zone 2, reducing total weekly volume capacity.

Signs Your Zone 2 Training Is Working

Aerobic adaptations from zone 2 training are gradual — expect 8-12 weeks before noticeable performance shifts. Track these markers to confirm progress:

• Cardiac drift reduction: Heart rate stays more stable during 90-minute+ easy runs. A drift of less than 5% from start to finish indicates strong aerobic fitness
• Pace at fixed heart rate improves: If you run at 135 bpm and your pace drops from 6:30/km to 6:00/km over 12 weeks, your aerobic engine is measurably stronger
• Morning resting heart rate decreases: A drop of 3-5 bpm over 8-12 weeks reflects increased stroke volume from consistent aerobic training
• Fat oxidation rate improves: If using metabolic testing, peak fat oxidation should shift to a higher absolute intensity. Practically, this means you can run faster while still burning predominantly fat

Fueling Zone 2 Sessions and Building Fat Adaptation

For zone 2 sessions under 90 minutes, training with minimal carbohydrate intake can deepen the fat-oxidation adaptation. This does not mean training fully fasted — a small protein-rich snack before the session protects against muscle breakdown without blunting the fat-oxidation signal. For sessions exceeding 90 minutes, begin fueling with 30-40g of carbohydrates per hour to prevent excessive glycogen depletion that would compromise the remainder of the training week.

On long runs of 3 hours or more, practise your race-day fueling strategy using NorthLine energy gels — the 22g of dual-transporter carbohydrates per gel matches the fueling demands of ultra-paced efforts without causing the GI distress that derails so many ultra-runners in the later stages of races. Building gut tolerance during zone 2 long runs is itself a training adaptation. Use the Race Day Nutrition Planner to calculate your carbohydrate and fluid targets for both training long runs and race day.