Training

The jump from marathon to 100K is not simply "more miles" — it is a fundamentally different physiological and logistical challenge. A marathon depletes glycogen stores over 3-4 hours; a 100K demands 8-14 hours of sustained forward motion where fat oxidation, gut tolerance, muscle durability, and psychological resilience become the limiting factors rather than VO2max or lactate threshold. The metabolic shift is dramatic: at marathon pace you burn approximately 70% carbohydrate and 30% fat, but during a 100K at the necessarily slower pace, the ratio flips toward 50-60% fat oxidation — which means training your aerobic system to burn fat efficiently is as important as building mileage.

A well-designed 24-week plan for a first 100K assumes a base of at least 50-60km per week and the ability to complete a 25-30km long run comfortably. If you cannot tick both boxes, spend 8-12 weeks building to these minimums before starting the plan. Rushing into ultra-distance volume on an insufficient base is the single most common reason first-time ultra runners develop overuse injuries — stress fractures, IT band syndrome, and Achilles tendinopathy account for 65-70% of ultra training injuries, and nearly all are preventable with gradual load progression.

24-Week Periodisation Structure

The plan divides into four distinct phases, each with a specific physiological purpose:

• Weeks 1-8 — Base Building: Weekly volume progresses from 60km to 80km. Long runs build from 25km to 35km. The focus is aerobic development at zone 2 (65-75% max HR). Run 5-6 days per week with one complete rest day. Include 1,000-1,500m of vertical gain per week if your goal race has elevation
• Weeks 9-16 — Specific Endurance: Weekly volume reaches 80-100km. Long runs extend to 35-45km. Introduce back-to-back long runs (e.g., Saturday 35km + Sunday 25km) every 2-3 weeks to simulate racing on fatigued legs. Vertical gain targets increase to 1,500-2,500m per week. Begin race-specific terrain training
• Weeks 17-21 — Peak and Race Simulation: Weekly volume peaks at 100-120km during weeks 17-19. Long runs reach 45-50km. Include one full race simulation at 60-70km (or a 50K race) during this phase. This block also serves as the primary window for dialling in nutrition, gear, and pacing strategy under race-like conditions
• Weeks 22-24 — Taper: Volume reduces by 40-60% across three weeks (week 22: 75% volume, week 23: 50%, week 24: 30%). Maintain intensity on two short sessions per week but eliminate all runs over 20km. The taper allows glycogen supercompensation, muscle tissue repair, and mental freshness

The Back-to-Back Long Run Protocol

Back-to-back long runs are the signature workout of ultra training and the session most specific to the demands of a 100K. Running 25-35km on tired legs from the previous day's 30-45km effort teaches your body to recruit muscle fibres that are normally held in reserve, improves fat oxidation efficiency, hardens connective tissue, and — critically — trains your gut to process calories during extended low-grade nausea:

• Frequency: Every 2-3 weeks during the specific endurance and peak phases (weeks 9-21). Total of 5-7 back-to-back weekends across the plan
• Day 1 volume: 30-45km at easy to moderate effort (zone 2-3, 65-80% max HR). Include race-pace segments of 10-15km in the second half
• Day 2 volume: 20-35km at easy effort only (zone 1-2, below 75% max HR). The goal is time on tired legs, not pace. Walk uphills if necessary — this mirrors what you will do in the final 30km of a 100K
• Recovery: Follow every back-to-back weekend with 2-3 easy days (30-40 min easy jogs or complete rest). The following week should be a reduced-volume recovery week (70-80% of normal volume)

Do not attempt back-to-back long runs before week 9. The connective tissue adaptation required to absorb this training stress takes 6-8 weeks of consistent base building to develop.

Vertical Gain and Terrain-Specific Training

Most 100K races include significant elevation — even "flat" ultras typically accumulate 1,000-2,000m of vertical gain over the course. Mountain 100Ks may include 4,000-6,000m of climbing. Your training must reflect this reality:

• Weekly vertical targets: Base phase 1,000-1,500m/week → Specific phase 1,500-2,500m/week → Peak phase 2,000-3,000m/week. If training on flat terrain, use a treadmill at 8-12% grade or stair repeats to accumulate vertical
• Uphill power hiking: At grades above 15-20%, walking is more energy-efficient than running for most athletes. Practice power hiking at 5-6 km/h on steep terrain — this skill saves 15-25% energy compared to running the same climb
• Downhill training: Eccentric muscle damage from downhill running is the primary cause of the severe quad fatigue that forces DNFs in the final 20-30km of mountain 100Ks. Dedicate one run per week to technical downhill training on trails with 8-15% grade. Build downhill volume gradually — eccentric muscle adaptation takes 3-4 exposures to establish protective resistance

Weekly Structure Example: Peak Phase (Week 18)

A typical peak-phase training week at 110km total volume:

• Monday: Rest or 30 min easy walk
• Tuesday: 15km easy with 4 × 8 min at tempo effort (zone 3-4). Include 400m vertical gain on hilly route
• Wednesday: 12km easy zone 2. Flat terrain, recovery focus
• Thursday: 18km with 600m vertical gain. Practice uphill power hiking and controlled descents
• Friday: 8km easy or rest
• Saturday: 40km long run on race-similar terrain. Practice full race nutrition protocol (60-90g carbs/hour). Include 1,200m vertical gain
• Sunday: 20km easy on tired legs (back-to-back). Zone 1-2 only. Walk all uphills

Total: approximately 113km with 2,200m vertical gain. This week would be followed by a recovery week at 75-80km.

Taper Protocol: The Final 3 Weeks

The taper is where first-time ultra runners most often make mistakes — either tapering too aggressively (losing fitness and gaining anxiety) or not tapering enough (arriving at the start line with residual fatigue). The evidence-based approach for ultra-distance events:

• Week 22 (3 weeks out): Reduce volume to 75% of peak (approximately 80-90km). Last long run of 25-30km. Maintain one moderate-intensity session (tempo intervals or hill repeats) to preserve neuromuscular sharpness
• Week 23 (2 weeks out): Reduce to 50% of peak (approximately 55-60km). Longest run 15-18km. Two easy runs of 8-10km. Focus shifts to sleep optimisation (8-9 hours per night), nutrition loading (8-10g carbohydrate per kg body weight in the final 48 hours), and gear finalisation
• Week 24 (race week): Reduce to 30% of peak (approximately 30-35km across Monday-Thursday). Two 10-12km easy shakeout runs. Complete rest Friday before a Saturday or Sunday race. Pre-race meal 3 hours before start: 600-800 kcal, primarily simple carbohydrates with moderate protein

Race Day Fueling and Pacing

Your pacing strategy for a first 100K should be conservative to the point of feeling easy for the first 40km. Target 70-75% of your marathon pace for the opening half. The second 50km is where the race begins — athletes who start even 5-10% too fast typically pay a 20-30% pace penalty after 70km. For nutrition, plan to consume 200-300 kcal per hour from a mix of gels, real food, and electrolyte drinks. Practice this intake during your long runs and race simulation — gut tolerance at hour 8 is entirely different from hour 2, and the only way to train it is to eat while running long.

Build your race-day nutrition plan around products you have tested extensively in training. NorthLine gels provide 22g of carbohydrate per serving with a dual-transporter formula — plan for 3-4 gels per hour combined with solid food (rice balls, boiled potatoes, or sandwiches from aid stations) for variety. Use the Race Day Nutrition Planner to map out your exact intake schedule by checkpoint, and calculate your fluid needs with the Sweat Rate Calculator before race week. For a comprehensive fueling strategy beyond gels, see our guide on ultra-marathon fueling.