24-Week Training Plan for a 100K Ultramarathon: From Mileage Building to Pre-Race Tapering

Introduction

Tackling a 100K ultramarathon is a milestone dream for many long-distance runners. This distance represents a “full 100 kilometers” challenge—typically 10–20 hours of trail running, spanning day and night, testing the limits of body and willpower. To perform well on race day, or even just to finish safely, requires a scientific and systematic training plan.

This article provides a 24-week training plan suitable for runners with marathon finishing experience who are preparing to take on their first 100K ultramarathon. The plan is divided into four phases, from base mileage building to pre-race taper, with clear training focuses and weekly mileage targets for each phase.

Prerequisites for the Training Plan

Before starting this plan, you should have:

• Marathon finishing experience (trail race experience is a plus)
• A current weekly mileage of at least 35–45K
• No major injury issues
• The ability to complete a single 25K long run
• The availability to commit 5–6 days per week to training

Overview of the Four Training Phases

Phase	Weeks	Focus	Target Weekly Mileage
Phase 1: Base Building	Weeks 1–6	Mileage accumulation, trail adaptation	50–70K
Phase 2: Volume Intensification	Weeks 7–14	Long-distance expansion, Back-to-Back	70–90K
Phase 3: Race Sharpening	Weeks 15–20	Quality training, race simulation	80–100K
Phase 4: Taper & Race Prep	Weeks 21–24	Mileage reduction, fitness maintenance	60→30K

Phase 1: Base Building (Weeks 1–6)

Training Focus

The core of this phase is “steadily increasing mileage.” Don’t rush into high-intensity training; first let your body adapt to a higher training load.

Weekly Training Structure (Example: Week 3):

• Monday: Rest or light cross-training (swimming, yoga)
• Tuesday: Easy run 8K, flat or gently rolling terrain
• Wednesday: Hill repeats (4×5-minute uphill efforts) + easy run 3K
• Thursday: Easy run 10K
• Friday: Rest or core strength training
• Saturday: Long run 25–28K, primarily on trails
• Sunday: Easy short run 8K (recovery run)

Cumulative Elevation Gain Target for This Phase: 600–1000m weekly gain

Key Workout: Hill Repeats

Schedule one hill repeat session per week:

• Find a climb with an 8–15% grade and 300–500m in length
• Run hard uphill, jog/walk easily back down
• Week 1: 4–6 repetitions; Week 6: 8–10 repetitions

Phase 2: Volume Intensification (Weeks 7–14)

Training Focus

This is the highest-volume phase of the entire plan. The focus is on building long-distance tolerance and getting your body accustomed to “running under cumulative multi-day fatigue”—which is precisely the core challenge of a 100K race.

Back-to-Back Long Runs:
Schedule 1–2 Back-to-Back long runs per month, for example:

• Saturday: 30–35K long run (trail, 1000m+ elevation gain)
• Sunday: 20–25K long run (continuing in a fatigued state)

This training pattern effectively simulates the late-race fatigue of a 100K, teaching your body to keep moving when muscles are tired.

Peak Week Training Structure (Weeks 10–11):

• Weekly mileage target: 88–95K
• Weekly elevation gain target: 2000–3000m
• Longest single run: 38–42K

Full Testing of Nutrition Strategy

During the long runs in this phase, begin systematically testing your race nutrition strategy:

• Use the race pack and gear you plan to race with
• Fuel according to your planned race frequency and food types
• Record gastrointestinal responses and adjust the strategy accordingly

Phase 3: Race Sharpening (Weeks 15–20)

Training Focus

Building on the high training volume, add targeted quality sessions to improve running efficiency and your ability to cope under fatigue.

Types of Quality Workouts:

1. Tempo Run: Once per week, 8–12K, maintaining a “comfortably hard” heart rate zone (approximately lactate threshold pace)
2. Marathon-Pace Long Run: Once per month, 15–20K, at 90–95% of your target race pace
3. Enhanced Hill Work: Increase the grade and number of hill repeats to strengthen glutes and core

Participating in a Simulation Race

Schedule a 50K trail race or training race during weeks 16–18 as a “dress rehearsal” for the 100K. The goal is not your finishing time, but rather to:

• Test your complete race gear and nutrition strategy
• Experience your mental state under race conditions
• Identify areas that need improvement

After the simulation race, give yourself at least 1.5–2 weeks of recovery before entering Phase 4.

Phase 4: Taper & Race Prep (Weeks 21–24)

Tapering Principles

The taper is the most important—and most easily sabotaged—training phase before race day. Many runners continue high-intensity training during the taper because they “feel too idle,” only to show up at the starting line with fatigued bodies.

Correct Tapering Strategy:

Week (Pre-Race)	Weekly Mileage	Notes
Week 21 (4 weeks out)	70–75K	Final long run (28–32K)
Week 22 (3 weeks out)	60K	Reduce long run to 20K
Week 23 (2 weeks out)	45K	Keep short runs, maintain feel
Week 24 (Race Week)	20–25K	Easy short runs, conserve energy

Mental Management During the Taper: Common “Taper Madness” symptoms—feeling heavy legs, decreased energy, unexplained fatigue—are all normal physiological and psychological responses, not signs of actual fitness loss. Trust your training accumulation and shift your focus to gear organization and race strategy planning.

Final Pre-Race Preparation Checklist

One Week Before the Race:

• Confirm the mandatory gear list and check each item
• Test headlamp battery life and prepare spare batteries
• Confirm the quantity and packaging of your nutrition supplies
• Prepare race clothing (including backup garments)
• Study the course map and aid station locations

The Day Before the Race:

• Easy jog for 15–20 minutes to keep the body active
• Confirm check-in time and start location
• Have a carbohydrate-focused dinner, finishing by 21:00
• Prepare all gear the night before to avoid a rushed early morning
• Aim for 8 hours of sleep (even if you can’t fall asleep, lying down and resting helps)

Practical Advice

Mileage Isn’t the Only Metric: In ultramarathon training, “elevation gain” and “time on feet” are equally important. Sometimes, a 4-hour trail run is more valuable than a 2-hour road run, even if the mileage is similar.

Listen to Your Body: The plan is a guideline, not an ironclad rule. If you need to cut back a week due to fatigue or minor aches, don’t push through. In ultramarathon training, adjusting one week barely affects overall preparation, but continuing to train through an injury can force you to stop for several weeks.

Record and Reflect: Keep a training log (using Strava, Garmin Connect, etc.), recording mileage, elevation gain, nutrition, and how you felt for each session. This data is the best basis for adjusting your training plan and valuable material for post-race review and improvement.

Conclusion

A 24-week training plan may seem long, but every week of accumulation is your most solid foundation at the 100K starting line. An ultramarathon is not just a race—it’s a 24-week journey of self-discovery and breakthrough. Follow the plan, step by step, and the finish line will be waiting for you there.

Related Reading

• Long-Term Planning for Ultramarathon Preparation: Mileage Accumulation Strategies Over 6 Months
• Complete Guide to Marathon Training Plans: The Progression Route from Half Marathon to Full Marathon
• Training Load for Ultramarathons: Recommendations for Weekly Mileage, Elevation Gain, and Long Run Distance
• First Marathon Preparation Plan: From 10K to Full Marathon in 12 Weeks