The Long Run, Solved: How Long Should Your Longest Run Be (Half & Full) + The Why
TL;DR (for the skimmers)
Half marathon: longest long run = 12–14 miles.
Full marathon: longest long run = 20–22 miles (or ~up to 3 hours, whichever comes first).
Weekly-volume rule: the long run should be ~25–30% (most def not more than 50%!!! for the love.) of your weekly mileage. If your week is 40 miles, the long run is ~10–12 miles; 60 miles up to about ~15–18 miles. *Note there is an exception here for marathon training that you can read in full detail below. :)
Protect yourself: avoid single-session spikes. Don’t suddenly run far beyond your longest run in the last 30 days— that’s linked with sharply higher injury risk.*
Fuel it: target 30–60 g carbs/hour (and >60 g/h once you’re out past ~2 hours). Practice on long runs.
Taper smart: reduce volume ~41–60% over ~2 weeks, keep intensity, keep frequency.
Why the long run works (the real physiology)
Long, steady running triggers the endurance adaptations we’re chasing: more and better mitochondria (little powerhouses!), improved capillary density, and shifts toward greater fat utilization so you spare glycogen late in the race. At the cellular level, endurance work upregulates signaling (e.g., PGC-1α) that drives mitochondrial biogenesis and vascular remodeling in muscle.
Those changes, repeated week after week, are what make holding goal pace feel more “automatic”, not a heroic one-off mega run. So patience here as we build truly is king!!!
How long should the longest long run be?
Half Marathon
12–14 miles hits the sweet spot: substantial endurance stimulus without compromising recovery or the rest of the week’s quality. For more advanced athletes running up to 16 miles is also an option but this is reserved for those who run closer to 50-60+ miles per week. Again noting the 30% long run volume general guidelines.
Marathon
20–22 miles works for most athletes; alternatively use a time cap and keep the longest long run to ~3 hours so the recovery cost doesn’t cannibalize (fancy word for ‘eats up your muscle’) your next 10–14 days.
Why not run the full marathon race distance or 24–26 beforehand? Because the marginal aerobic gain beyond ~3 hours is small while muscle damage, glycogen depletion, and neuromuscular fatigue skyrocket — which can derail several key workouts that matter more.
The weekly-volume rule (your guardrails)
A durable guideline is: keep the long run to ~25–30% of your weekly mileage.
Examples:
30 mi/wk: ~7.5–9 miles
40 mi/wk: ~10–12 miles
50 mi/wk: ~12–15 miles
60 mi/wk: ~15–18 miles
This keeps stress proportional so you can absorb it and show up for the rest of the week.
*Running lower mileage? Use time instead of miles (e.g., 90–150 minutes) to prevent the long run from dwarfing your week.
*A Note on the “30% Rule”
Most of the time, the long run = 25–30% of weekly mileage is a solid guideline.
At around 40 miles per week, that usually means a 10–12 mile long run.
But marathon training makes an exception:
To prepare for 26.2, it’s common to stretch beyond that ratio in peak weeks.
Even on 40 miles per week, you may build toward 16–18 miles and include one carefully placed 20 miler.
The purpose is to rehearse fueling, pacing, and the mental durability that only comes from extended time on your feet.
The safeguard is progression and recovery: those longer runs are built gradually and paired with cutbacks to keep you healthy and consistent.
Progress without breaking: avoid single-run spikes
Newer research suggests injury risk is less about ‘neat’ week-to-week percentages and more about singular, outlier runs. If one session is >10% longer than your longest run in the prior 30 days, injury risk climbs; doubling it is worst-case.
Translation: build gradually and respect your recent ceiling.
Fueling & pacing your long run (practice now, race easy later)
Fuel
During: 30–60 g carbs/hour for 1–2.5 h; go >60 g/h (up to ~90 g/h with multiple transportable carbs) when you’re out past ~2 h. Practice gut training.
Before/after: arrive fueled (1–4 g/kg carbs 1–4 h pre-run), then replenish carbs + some protein to speed glycogen restoration.
Pace
Keep most long runs easy-to-steady (aerobic). Sprinkle in short marathon-pace segments in select long runs to rehearse fueling, cadence, and effort. Not every week.
A note for those gearing up for their first marathon or half at Asheville 2026
We’re building durable durability, not one heroic Sunday. With a hilly course, we’ll bias toward time-on-feet, strength on climbs, and smart caps so you’re peaking, not cooked. A simple scaffolding:
HALF (12–14 week arc)
Weeks 1–4: long run to 90–105 min (or ~9–11 mi), strides + gentle hills
Weeks 5–8: extend to 12–14 mi every other week; include 10–20 min at HM effort late in a few runs
Weeks 9–10: hold volume, sharpen (shorter, quicker workouts)
Weeks 11–12: 2-week taper (-41–60% volume), keep strides/short efforts.
FULL (18–20 week arc)
Base: long run 90–120 min, build to 2:15–2:30
Peak: 2–3 long runs 18–22 mi (or up to ~3:00), with select marathon-pace blocks
Protect the week: long run stays ≤30% of total mileage
Taper: ~2 weeks, volume down 41–60%, intensity/frequency maintained.
Mileage vs. Time on Feet
When it comes to the marathon long run, both distance and duration matter.
Athletes who naturally cover miles more quickly can often complete runs of 20–22 miles within that safe ~3-hour window.
Athletes who take longer to cover the same ground may cap the long run at 18–20 miles, so that time on feet doesn’t drift far beyond three hours.
The principle is the same for everyone: it’s not about proving you can run 26.2 in training: it’s about reaching the start line healthy, confident, and fully recovered from your biggest long runs.
Common mistakes I see (and how we avoid them)
Letting the long run swallow the week. A 16-mile Sunday off a 30-mile week is a red flag. We rebalance.
Chasing distance PRs every single week. Respect the 30-day longest-run ceiling to dodge injury spikes.
Under-fueling training. You wouldn’t race fasted, don’t “train” your bonk. 30–60 g/h (or more past 2 h).
Fueling Note: Fasted Runs vs. Carbs
Yes, there are some studies that suggest running fasted can help your body become more “fat adapted.” The idea is that without carbs, your body learns to lean more on fat stores for energy.
But here’s the reality: you don’t need to starve yourself to build endurance. Long runs are already the ultimate stimulus for fat adaptation. Skipping fuel only raises stress, undercuts quality, and delays recovery.
So my advice is simple: eat the carbs. Your body needs them to train well, recover well, and show up again tomorrow. For a more personalized approach reach out to Kendra our RD.
Skipping the taper. Performance gains land when volume drops (but we still keep some spunk in the step) and freshness rises.
Final word of advice from your coach
Your long run is a tool, not a test. We’ll use it to stack adaptations, protect your week, and get you to your marathon confident, tapered, and hungry.
If you’re ready to go all in submit your form here or if you know you’re running the Asheville Marathon check out our team here.
Sources:
How Much Running is Too Much: British Journal of Sports Medicine+1
Muscle breakdown Runner's World
