Marathon Training Plan Length 12, 16, or 20 Weeks

Pick 12 weeks if you're advanced, already running 15-20 miles a week, with a marathon or strong half in the last 12 months. Pick 16 weeks if you're intermediate, with half-marathon experience and a 25-35km/week base. Pick 20 weeks if you're a beginner, or you haven't raced a half marathon in the last year. Below: the three factors that actually decide your number (mileage gap, recent race history, injury history), what to do when your race date doesn't line up with a clean 12/16/20-week window, and what structurally separates a beginner, intermediate, and advanced plan.

Pick a plan that's too short for your current base, and the compressed peak week is usually where the injury shows up — not the marathon itself. The three-factor framework below is the same one I run through with BRT athletes before I'll sign off on a plan length, so you can work out your own number instead of guessing off a chart.

What Actually Determines Your Number

The table above is a starting point, not the full answer. Three factors decide your real number, and they matter in this order.

First, and most heavily weighted: the gap between your current weekly mileage and the peak-week mileage your target plan expects. The bigger that gap, the longer your build needs to be — you can't jump from 20 miles a week to a 40-mile peak week in 16 weeks without inviting injury. Second: how recently you've finished a half marathon or marathon. Within the last 12 months, your body still holds a lot of that fitness, and you can compress. Third: injury history. Any soft-tissue injury in the last 12 months adds four weeks to your plan, non-negotiable.

That third one is a rule I don't bend. Injured tissue needs graded loading time that a compressed plan simply can't give it — no amount of enthusiasm changes the biology.

Here's how that plays out in practice. A runner I coached was sitting at a 20 mile-a-week base and wanted a 16-week plan for a fall race. On paper, fine. But their target plan's peak week called for 40 miles — double their current base, inside 16 weeks. I moved them to 20 weeks with a lower peak-week target instead. Slower progress, same finish line, far less risk.

When Your Race Date Doesn't Give You the "Right" Number of Weeks

Real race dates rarely line up with a tidy 12, 16, or 20-week window. When you've got fewer weeks than the framework recommends, don't shorten your peak long runs to make the calendar work — compress by cutting base-building weeks instead, and cap your peak weekly mileage lower than a full-length plan would target. Protect the last three peak weeks always; that's where marathon-specific fitness actually gets built.

When you've got more weeks than you need, resist the urge to start the "real" plan early just to fill the time — that's a fast route to mid-cycle burnout. Instead, add a second build cycle with a step-back week in the middle, so you arrive at the true taper fresh rather than fatigued from months of unnecessary loading.

Beginner vs. Intermediate vs. Advanced: What Actually Differs

The framework above assumes you know what separates these tiers structurally. Here's the short version.

Once you've picked your length, head back to the full marathon training plan hub to find the right timeline page for your race date, or check your readiness benchmarks first if you're not sure you should start yet. If your plan length depends partly on how cross-training fits your week, that's covered separately at fitting cross-training into any plan length.

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