How Long Does It Take to Train for a Half Marathon?

Most runners need 10 to 20 weeks to train for a half marathon. Where you start sets the number. The half marathon is 13.1 miles. The thing that carries you there is the long run. It grows slowly, week by week. That slow growth is why the half needs a real runway, and why you cannot safely rush it.

Starting fitness	Weeks to half-ready	Long-run build
New to distance, can jog a 5K	16–20 weeks	Grow the long run from 3–4 mi up to 10–12 mi
Can run a 10K in comfort	10–14 weeks	Grow the long run from 6–7 mi up to 11–12 mi
Experienced, long run already 8+ mi	8–10 weeks	Hold the long run near 10–12 mi, then sharpen

How long does it take to train for a half marathon?

Most runners need 10 to 20 weeks. Your base picks the number. Find your row in the table above.

A half marathon is 13.1 miles, or 21.1 km. It is a true endurance race. You will be on your feet for one to three hours. That is a long time. So your timeline does not turn on speed. It turns on one number: how far your long run can go right now.

This is what makes the half different from a 5K or a 10K. A short race can lean on grit for a few minutes. A half cannot. You need the miles in your legs first. The rest of this guide shows you how the long run sets your clock, and how few weeks you can safely use.

Why is the long run the spine of half marathon training?

The long run is the single most important session in half training. It is the one run that teaches your body to last 13.1 miles. Every plan is built around it.

Top coaches say so plainly. In the Hal Higdon half marathon plans, the long run is called "the key to the program." It builds from about 4 miles in week one to 12 miles near the end (Hal Higdon). That is roughly one mile added each week, with easy weeks mixed in. The long run grows. The race comes after it peaks.

12 mi

how far the longest training run typically reaches before a half marathon (Hal Higdon)

The reason is your aerobic engine. The long run builds your Aerobic Base, the slow-grown fitness that lets you hold a pace for miles. This base does not appear in a week. It rises over weeks and months of steady running (Jones & Carter, 2000). The research agrees that the long run matters for the half. A large study of half marathon runners found that the longest training run was tied to race performance (Fokkema et al., 2020).

So your timeline is really a question about the long run. How far can it go today? How far must it reach? The gap between those two numbers is your training runway.

How long if you can already run a 5K?

If a 5K is your current limit, plan for 16 to 20 weeks. You are not starting from zero. But you have a long way to grow your long run.

Your long run might be 3 or 4 miles now. For a half, it needs to reach 10 to 12 miles. That is a big jump. It cannot be rushed. You add a little distance to your long run each week, then ease back every third or fourth week. A 20-week plan gives that build room to breathe (RunToTheFinish).

Keep almost all of these miles slow and easy. Easy running is what builds your Aerobic Base. It also keeps you healthy enough to train week after week. If you are brand new to distance, spend your first weeks just building easy running before any half plan. The guide to building an aerobic base before a plan begins shows how. And if a half feels far off, the 10K is the smarter next step first.

How long if you can already run a 10K?

If you can finish a 10K in comfort, plan for 10 to 14 weeks. This is the sweet spot most half plans are built for.

Your long run is likely 6 or 7 miles already. You need to stretch it to 11 or 12. That is a shorter climb than the 5K runner faces. Most popular plans land right here. A review of half marathon plans found that the majority last 10 to 14 weeks (Runner's World).

Your week has a simple shape. One long run that grows. One faster session for sharpness. Two or three easy runs to hold your base. The long run does the heavy lifting. The faster work lifts your race pace. Keep building your weekly miles in small steps, and let the long run lead the way.

How long does an experienced runner need?

An experienced runner needs only an 8 to 10 week block. You already own the long run. Now you tune it for race day.

If you run four or five days a week and your long run is already 8 miles or more, you do not need to build endurance. You need to sharpen it. Hold your long run near 10 to 12 miles. Add quality, such as some miles at goal pace inside the long run. Then taper for the last week to arrive fresh.

For this runner, more weeks rarely mean a faster half. The engine is built. The gains now come from the right hard work and good rest, not from more time. That is why the experienced clock is the shortest of the three. To see what truly moves your time at this stage, read why peak weekly mileage will not save your half marathon.

What is the minimum safe runway for a half marathon?

The honest minimum is about 12 weeks for most people. Less than that, and you start trading safety for speed.

Coaches agree on this floor. Most advise at least 12 weeks, and they warn that less time raises the risk of burnout and injury (Outside). An 8 week plan can work, but only for a runner with a base of 15 to 20 miles a week (Still I Run). That runner is sharpening, not building.

12 weeks

the minimum most coaches advise to train safely for a half marathon (Outside)

The danger of rushing lives in the long run. A big jump in your longest run is exactly what hurts runners. A 2025 study of 5,205 runners found that a single run more than 10 percent longer than your recent longest sharply raised injury risk (Frandsen et al., 2025). The half tempts you to take that jump, because you must reach 13.1 miles. A short runway forces the jump. A long runway lets you grow the long run in safe steps.

This is also why steady building beats a frantic push. Sudden spikes in training load drive injury more than the load itself (Gabbett, 2016). The driver that protects you here is Load Consistency. Keep your weekly miles climbing in small, steady steps. Take an easier week every third or fourth week. Steady is not slow. Steady is what gets you to the start line healthy.

How does PIRX tell you how long you really need?

A generic plan picks your timeline from your age or your goal. PIRX reads your real runs and shows where your long run actually stands.

This matters for the half more than any other distance. Your timeline hinges on your long run and your Aerobic Base. A one-size-fits-all calculator cannot see either one. It sees a finish time and hands back a number. A wrist VO2 max reading is just as blind. It cannot tell you how far you can safely run next week.

PIRX is a proprietary machine-learning prediction engine. It learns from your training history, not a fixed formula. It scores five drivers: Aerobic Base, Threshold Density, Speed Exposure, Load Consistency, and Running Economy. For the half marathon, your Aerobic Base shows how much long-run growth you still need. Your Load Consistency shows whether you are building that growth safely or spiking toward injury.

PIRX turns this into a Projected Time with a Supported Range, so you see the spread of likely finishes for your fitness today. It recalculates after every synced activity. A visible change shows only when the structural change is at least 2 seconds. So you watch your long run grow into real seconds, not noise. Curious how your finish compares to others your age? See what counts as a good half marathon time by age.

98%

validated prediction accuracy (PIRX users)

See how many weeks your half really needs

Stop guessing your half marathon timeline. Generic plans treat every runner the same. PIRX does not. Connect your Garmin, COROS, or Strava in under a minute. See your Aerobic Base and Load Consistency scores, find where your long run truly stands, and get a Projected Time with a Supported Range for your half. Then build the runway that fits you, not a stranger. Free, no signup needed.

Sources

• Hal Higdon. Half Marathon Training: Intermediate 1. (The long run is the key to the program, building from about 4 miles to 12 miles across a 12-week plan.)
• Jones, A.M. & Carter, H. (2000). The Effect of Endurance Training on Parameters of Aerobic Fitness. Sports Medicine, 29(6), 373-386. (Aerobic gains build through steady endurance training over weeks and months, not days.)
• Fokkema, T. et al. (2020). Training for a (half-)marathon: Training volume and longest endurance run related to performance and running injuries. Scand J Med Sci Sports. (The longest training run was linked to half marathon performance; very high volume also raised injury risk.)
• Frandsen, J.S.B. et al. (2025). How much running is too much? Identifying high-risk running sessions in a 5200-person cohort study. British Journal of Sports Medicine. (A single run more than 10 percent longer than your recent longest sharply raised injury risk.)
• Gabbett, T.J. (2016). The training-injury prevention paradox: should athletes be training smarter and harder? British Journal of Sports Medicine. (Sudden spikes in training load drive injury; build gradually.)
• Outside. A Half Marathon Training Plan for Beginners: 20 Weeks. (Most people need a minimum of about 12 weeks; less time raises the risk of burnout and injury.)
• Still I Run. 15 Free Half Marathon Training Plans for Beginner, Intermediate, and Advanced Runners. (Ideal plans run 12 to 16 weeks; an 8-week plan suits runners already on 15 to 20 miles a week.)
• Runner's World. The Best Half Marathon Training Plans for Every Level of Runner. (The majority of half marathon plans last 10 to 14 weeks, with longer plans for beginners.)
• RunToTheFinish. Couch to Half Marathon: 20 Week Plan. (A 20-week build suits runners new to distance who must grow the long run gradually.)