Half Marathon Training Plan: 12 Weeks to Race Day

Structured 12-week half marathon training programme for UK runners with varying fitness levels

Why 12 Weeks Is the Sweet Spot

Most runners who sign up for a half marathon give themselves either too much time (lose motivation by week 14) or too little (panic-train for 6 weeks and get injured). A 12-week half marathon training plan hits the middle ground: long enough to build fitness safely, short enough to stay focused.

I’ve coached runners through dozens of half marathons, and the pattern is always the same. The ones who follow a structured 12-week programme finish stronger and enjoy the experience more than those who wing it. Here’s the plan I keep coming back to.

Before You Start: Where You Need to Be

This plan assumes you can comfortably run 5K (about 30-40 minutes) without stopping. If you’re not there yet, spend 4-6 weeks building up to that baseline first. There’s no shame in it. Rushing into half marathon training on a weak foundation is how shin splints happen.

You’ll also need three days per week for running, plus one day for cross-training. That’s it. No six-day-a-week grind. Quality over volume is the entire philosophy here.

The Three Runs That Matter

Every week in this plan revolves around three key sessions. Each one serves a different purpose, and skipping any of them regularly will show up on race day.

The Easy Run (Tuesday) — Conversational pace. If you can’t hold a full sentence, you’re going too fast. These runs build aerobic fitness without battering your legs. Most runners do these too hard, which is the single biggest training mistake I see.

The Quality Session (Thursday) — This is where the improvement happens. Tempo runs, intervals, or hill repeats, depending on the week. These sessions teach your body to run faster at a lower effort level.

The Long Run (Sunday) — The cornerstone of half marathon training. You’ll build from 8K up to 18K over the 12 weeks. Run these 30-60 seconds per kilometre slower than your target race pace. The goal is time on your feet, not speed.

Half Marathon Training Plan: 12-Week Schedule

Phase 1: Base Building (Weeks 1-4)

The first month is about consistency, not heroics. You’re laying the groundwork.

Notice week 4 drops the volume. Every fourth week is a recovery week. This isn’t laziness. It’s how your body actually adapts. The fitness gains happen during recovery, not during the hard sessions.

Phase 2: Building Strength (Weeks 5-8)

Now the mileage starts climbing and the quality sessions get more specific.

By week 7, your long run hits 15K. That’s the point where most runners start believing they can actually do this. Trust the process. If 15K feels manageable at an easy pace, 21.1K on race day with adrenaline and crowd support is absolutely within reach.

Phase 3: Race Preparation (Weeks 9-12)

Peak training, then taper. This is where everything comes together.

Week 10 is your peak week. The 18K long run is the furthest you’ll go before race day. After that, you taper: reduce volume by 25-30% in week 11 and significantly in week 12. You’ll feel restless. You’ll wonder if you’re losing fitness. You’re not. Your body is absorbing twelve weeks of work and getting ready to perform.

Pacing: The Mistake That Ruins Half Marathons

The number one reason runners hit the wall at mile 10 is going out too fast. It feels easy at 3K. It does not feel easy at 16K.

Work out your target pace before race day. If you can run 5K in 27 minutes, a realistic half marathon target is around 2:05-2:10. If your 5K is 24 minutes, aim for 1:50-1:55. These aren’t exact, but they’ll stop you from doing something stupid in the first mile.

Run the first 5K at your target pace or slightly slower. Pick it up in the second half if you’re feeling strong. Negative splits (running the second half faster than the first) are how good half marathons happen.

Fuelling Your Training

For runs under 75 minutes, water is enough. For your longer runs (12K+), start practising with energy gels or sweets. Race day is not the time to discover that a particular gel brand makes you feel ill.

A general rule: take on 30-60g of carbohydrate per hour for efforts over 90 minutes. That’s roughly one gel every 30-40 minutes. Practise this in training. Your stomach needs conditioning just like your legs.

The night before long runs, eat a carb-heavy meal. Pasta, rice, potatoes. Nothing exotic, nothing you haven’t eaten before. Same principle applies to race week.

Cross-Training and Rest

One cross-training day per week makes a significant difference. Swimming, cycling, or a strength session focusing on glutes, core and single-leg stability. According to the British Journal of Sports Medicine, runners who include strength training reduce their injury risk by up to 50%.

Rest days are non-negotiable. Two per week minimum. If something hurts beyond normal muscle soreness (sharp pain, pain that gets worse as you run, pain that changes your gait), take an extra rest day. Missing one run is nothing. Missing six weeks with an injury is everything.

The Common Mistakes

Running easy days too fast. This is so common it bears repeating. If you’re not running easy days slowly enough, you’re too tired to run quality sessions properly. The result: mediocre training across the board instead of genuine improvement.

Skipping the taper. “I’ll just do one more long run in week 11.” Don’t. The taper works. Respect it.

New shoes on race day. Wear them in during training. At least 50K in your race shoes before the event.

Ignoring the weather. UK races in spring and autumn can throw anything at you. Check the forecast the night before and have layers ready. A bin bag with arm holes makes a surprisingly effective throwaway rain jacket for the start line.

Finding Your Race

The UK has hundreds of half marathons throughout the year, from flat city courses to hilly trail events. Browse the UK race calendar to find one near you. Popular options include the Great North Run, Bath Half, and the Royal Parks Half in London.

If you’re training with a club, the accountability makes a massive difference. Most running clubs across the UK have members training for the same events, which means built-in training partners for those long Sunday runs. If you don’t have a club yet, check what’s available in your city and try a few sessions.

Race Week: Keep It Simple

Monday to Thursday of race week: one short easy run of 3-4K on Tuesday or Wednesday. That’s it. Eat normally, sleep well, hydrate.

Friday: lay out your kit. Everything you’re wearing on race day should be something you’ve trained in. Pin your number on tonight so you’re not faffing around at 6am.

Saturday: rest. Light walk if you’re restless. Carb-heavy dinner. Early to bed.

Sunday: you’ve done the work. Trust the training, stick to your pace plan, and enjoy it. The last 3K will hurt regardless of how well you’ve trained. That’s just what half marathons do. Push through it. The finish line feeling is worth every Tuesday morning alarm.