How to Train for a Half Marathon in 12 Weeks
A 12-week training block is plenty of time to prepare properly for a half marathon — provided you already have a reasonable running base.
You don’t need to be running huge mileage before you start, but I’d ideally want an athlete to be comfortably capable of running 10k in training and already running two or three times per week.
From there, the aim isn’t simply to run more and more every week. A good half marathon plan should gradually build your endurance, introduce the right amount of quality work and make sure you’re fresh enough to actually perform those harder sessions properly.
Here’s how I’d approach a 12-week half marathon build.
Who is a 12-week half marathon plan suitable for?
Before starting, I’d ideally want you to be:
Running 2–3 times per week
Comfortable completing 10k as a normal training run
Generally consistent with your running
Free from any current injury problems that are stopping you training normally
You absolutely don’t need to be capable of running 13.1 miles before starting the programme.
That’s what the 12 weeks are for.
The goal is to progressively bridge the gap between the running you can comfortably handle now and what you’ll need on race day.
How many times per week should you run for a half marathon?
For most athletes, I think three runs per week is absolutely enough to prepare well for a half marathon.
More isn’t automatically better.
With three well-planned sessions, you can cover the key areas without simply adding mileage for the sake of it.
As the programme becomes more race-specific, some athletes may move to four runs per week, but there needs to be a reason for adding that fourth run.
A typical week might contain:
An easier aerobic run
A quality or threshold-based session
A longer endurance run
During the more specific race block, an additional easy or race-specific run can be introduced where appropriate.
The key is how those sessions work together.
What is the most important half marathon training session?
Every session has a purpose, so I’m always reluctant to label one workout as the most important.
However, if there is one session that needs to be built correctly, it’s probably the long run.
The mistake is thinking that a long run is simply about making the distance longer every Sunday.
It should progress alongside the rest of your training.
Early in the programme it may simply be about comfortable aerobic endurance. As you become fitter, parts of the long run can become more specific to the demands of your race.
That progression is far more useful than repeatedly slogging through long runs at the same intensity.
How long should your longest run be before a half marathon?
You do not need to run the full 13.1 miles in training before race day.
For most athletes, I think somewhere around 9–11 miles is absolutely fine for the longest run of the build.
By that stage, you should have accumulated enough consistent training that the additional distance on race day isn't a huge leap.
There are obviously individual differences. A more experienced runner may benefit from slightly different training than somebody preparing for their first half marathon.
But I wouldn't judge the success of a half marathon programme purely by whether you've run 13 miles before the race.
Your whole training block matters.
Don't make every run hard
This is probably the biggest mistake I see.
A lot of runners spend nearly all of their training in the same middle-to-hard intensity zone.
Their easy runs aren't particularly easy, but they're also not hard enough to create the stimulus we'd want from a proper quality session.
The result?
When the genuinely important harder session arrives, they're already carrying too much fatigue to execute it properly.
That’s why training intensity matters.
Some sessions should feel comfortable.
Some should be challenging.
And occasionally you should be doing work that is specifically designed around the intensity you'll experience on race day.
If everything feels hard, something is normally wrong.
Easy running has a purpose
Easy training isn't wasted training.
Those runs allow you to build aerobic fitness and overall training volume without constantly adding excessive fatigue.
They also help you arrive at your harder sessions capable of actually doing them properly.
That distinction becomes increasingly important as your weekly training increases.
If you've got a threshold session scheduled on Thursday, turning Tuesday's easy run into an unofficial race isn't going to make Thursday better.
Usually, it does the opposite.
How should the 12 weeks progress?
I wouldn't think of a half marathon plan as twelve identical weeks with slightly more mileage added each time.
There should be a progression.
Early weeks
Build consistency and establish your routine.
The focus is on developing the aerobic base and gradually increasing what your body can tolerate.
Middle of the programme
The training becomes more progressive.
Long runs develop, quality sessions become more purposeful and overall race fitness starts to build.
Race-specific block
This is where training starts looking increasingly like the demands you'll face on race day.
For some runners, this may also be where a fourth weekly run becomes useful.
Final weeks
Training volume begins to reduce so that fatigue comes down without losing the fitness you've spent the previous weeks building.
You don't gain much by reaching race week exhausted.
The goal is to arrive at the start line fit and fresh.
What happens if you miss a session?
Don't panic and don't immediately try to cram missed sessions back into the week.
One missed run will have almost no meaningful effect on a 12-week programme.
Trying to compensate by stacking several sessions together can.
Consistency across the whole training block matters far more than completing every single workout perfectly.
A good plan should give you structure without making you a slave to the calendar.
The aim: arrive ready to race
A successful half marathon build isn't about completing the biggest possible mileage.
It's about putting together 12 weeks of consistent, progressive training where every type of session has a purpose.
Build the long run correctly.
Keep your easier running genuinely controlled.
Hit the harder sessions when you're fresh enough to benefit from them.
Then give your body enough recovery to absorb the training.
Do that consistently and you'll arrive on race day in a very different place from where you started.
Want the complete 12-week programme?
My 12-Week Half Marathon Training Plan gives you the full training structure delivered directly through TrainingPeaks.
You'll get progressive running sessions, long runs, quality work, recovery and a structured taper into race day.