What each distance actually is
The half marathon is 21.0975 km, or 13.1 miles. The marathon is exactly double — 42.195 km, or 26.2 miles. They share branding, signage, expos, and finish-line celebrations. They do not share physiology. Treating the marathon as “two halves stacked” is the single most common mistake in distance running.
For most recreational runners, a half takes 1:45-2:30. A full marathon takes 3:45-5:30. The faster you go, the more the time gap compresses — but the suffering gap widens.
Why the second half of a marathon is a different sport
Glycogen is the central character. Your body stores roughly 2,000 calories of carbohydrate as muscle and liver glycogen. A 70 kg runner burns about 70 calories per kilometer at moderate effort. That’s 1,470 calories for a half marathon — well within stores. For a full marathon, it’s 2,950 calories — meaningfully more than you can carry.
This is the math behind “the wall.” Around kilometer 30-35, glycogen-depleted runners shift hard toward fat oxidation, which produces ATP more slowly. Pace drops. Form crumbles. The final 10K of a marathon often takes 25-40% longer per kilometer than the first 10K, even for trained athletes who paced themselves correctly.
The half marathon largely avoids this. You can run a half on poorly stocked glycogen and survive. You cannot do the same in a full marathon.
Training looks different too
A half marathon plan typically runs 10-14 weeks with peak weekly volume of 40-60 km. The longest training run is 16-18 km — shorter than the race itself, because the race is short enough that you don’t need to fully simulate it.
A marathon plan runs 16-20 weeks with peak weekly volume of 60-100 km. The longest training run is 30-35 km, and most plans ask for 3-4 runs at that length. The cumulative mechanical load is dramatically higher, which is why marathon prep carries a meaningfully higher injury rate than half-marathon prep.
When to pick the half marathon
- It’s your first race over 10K. The half is the canonical first long-distance race. It’s serious enough to demand respect but short enough to be safe.
- You have 8-10 hours per week for training. Half marathon prep fits real-life schedules. Marathon prep often doesn’t.
- You want to race more than once a year. Recovery from a half is 1-2 weeks. You can race 4-6 halves per year without burnout.
- You’ve had recent injuries. The lower training volume keeps tissue load manageable.
- You enjoy running fast. Half marathons are run at a higher percentage of VO2max than marathons, so they reward speed and threshold work more directly.
When to pick the marathon
- You’ve finished 2-3 half marathons. You understand pacing, fueling, and what 2+ hours of running feels like.
- You can commit to 16-20 weeks. Marathon training is non-negotiable on the long-run schedule. Missing the 30km long run is not optional.
- You want the bucket-list achievement. Crossing a marathon finish line is a singular life experience. There is no half-marathon equivalent.
- You’re chasing a Boston-qualifying time or another major-marathon entry standard. That’s a marathon-specific goal — see our Boston BQ calculator.
- You enjoy the meditative long run. The 3-hour Sunday training run is the heart of marathon prep. If that sounds like a chore, the marathon may not be your race.
Common misconceptions
“If I can run a half, I can run a full.” Doubling distance is not a linear ask. Predictors suggest marathon time ≈ half marathon time × 2.1 to 2.2, but that assumes adequate marathon-specific training. Without it, the multiplier blows out to 2.4 or worse.
“The wall is mental.” The wall is physiological. It’s glycogen depletion plus accumulated mechanical fatigue. Mental toughness helps you keep moving through it; it does not prevent it.
“Recovery from a marathon is just a few rest days.” The cellular damage from 42 km of running takes 3-6 weeks to fully clear, even if you feel fine after a week. Racing again too soon is how runners enter chronic-injury territory.
“Marathons are mostly about mileage.” They’re about specific mileage. A high-volume runner who never practices marathon pace will run a slower marathon than a moderate-volume runner who does.
Recommendation
If you’ve never raced beyond 10K, pick the half marathon. Do two of them. Learn what carb-loading, gel-fueling, and 90-minute-plus pacing feel like in your body. Then decide whether you want to step up.
If you’ve finished a couple of halves, can train consistently 5 days a week, and have 16-20 free weeks, pick the marathon. Use a structured plan. Don’t skip the long runs. Practice your race-day fueling on every run over 90 minutes, not just on race day.
If you’re torn, the math favors the half: better training-to-result ratio, lower injury rate, more frequent racing opportunities. The marathon should be a deliberate choice, not a default.