How to set a realistic marathon finish time goal

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Picking a marathon goal time is one of those things that sounds straightforward until you’re actually doing it. You search online, find a finish time predictor, plug in your 5K PB from three years ago, and it spits out 3:52. But is that realistic? Should you aim for it? What if you’ve been running four days a week and also working full-time and occasionally sleeping badly because life is life?

This article is for runners who want a goal that means something — not a number pulled from a calculator or borrowed from a training partner who runs 70km a week. A good marathon goal is one you can actually train for, given the time you have, the fitness you’re starting with, and the fact that things will go wrong between now and race day.

Here’s how to find yours.

Start with what you’ve actually run lately — not your best ever

The most common mistake runners make when setting a marathon goal is reaching back to their peak fitness. Your 22-minute 5K from two years ago, before you changed jobs, had a kid, or went through a rough patch — that’s not your current data. It’s history.

What you want is a recent race or time trial result — ideally something from the past six to twelve weeks. A parkrun time, a 10K, even a strong half marathon if you’ve raced one. These give you a real baseline. If you haven’t raced recently, do a time trial: run 5K as hard as you can, properly warmed up, on a flat course, and use that.

From a recent honest effort, you can start projecting forward — but the projection still needs adjusting for what your marathon training actually looks like.

Use a predictor — but don’t trust it blindly

Race time predictors are useful starting points. The most widely referenced is Riegel’s formula, which estimates that as race distance doubles, pace slows by roughly 6%. Most online calculators use a version of this. Plug in a 25-minute 5K and it’ll suggest something around 3:50–3:55 for the marathon.

The problem: Riegel’s formula assumes a well-trained runner with solid endurance. If your long run peaks at 25km rather than 35km, or you’re running three days a week rather than five, that prediction will flatter you. The marathon is not just a long race — it’s a metabolic challenge, and undercooked aerobic base shows up brutally after kilometre 32.

Use predictors as a ceiling, not a target. Then adjust down based on:

• How many days per week you train (3 vs. 5 makes a real difference)
• Your peak long run distance and how you handle it
• Whether your weekly mileage has been consistent for 8+ weeks
• Your experience — first marathons rarely go to plan, even for fit runners

The honest pace table: matching recent times to realistic marathon goals

This table gives realistic marathon finish times based on recent race results, for runners following a moderate training plan (3–4 runs per week, long run up to 30–32km). If you’re doing more volume, you might nudge the top end. If this is your first marathon, lean toward the conservative column.

Conservative = first marathon or limited training volume. Moderate = second marathon or solid consistent base of 40–50km/week.

Factor in your training reality, not your training ideal

A training plan on paper looks neat. Your actual life is messier. Most recreational runners training for a marathon fit it around jobs, family, and the random Wednesday when everything goes sideways. That’s not a failure — it’s just the reality of running as an adult who has other obligations.

When you’re setting your goal, be honest about what your training will actually look like:

• If you can run 4 days a week, every week, reliably — you can aim toward the moderate column in the table above.
• If you’re fitting in 3 runs and they sometimes get cut short — the conservative column is your friend, and it’s still a real achievement.
• If you’ve never run more than 25km in a single run — don’t target a pace that requires 30km-plus of long runs you’ve never done.
• If you’re injury-prone — build a buffer. Aiming 10–15 minutes beyond your predicted time gives you room to stay healthy rather than constantly chasing.

Running 42.2km is genuinely hard regardless of the time on the clock. A 5:00 finish for someone who’s only been running for a year is every bit as legitimate as a 3:00 finish for someone who’s been at this for a decade.

Half marathon as the most reliable predictor

If you want one number to anchor your marathon goal, it’s your half marathon time. It’s close enough to marathon fitness to be meaningful, and far enough that it requires real aerobic capacity. Research published in sports science consistently shows the half marathon is the strongest single-distance predictor of marathon performance.

A rough but practical rule: double your half marathon time and add 10–20 minutes.

• 1:50 half marathon → roughly 3:50–4:00 marathon
• 2:00 half marathon → roughly 4:10–4:20
• 2:15 half marathon → roughly 4:40–4:55

Add more buffer if it’s your first marathon, if you trained conservatively, or if the course has significant elevation. Subtract a little if you’ve done marathons before and negative splitting is genuinely a strength of yours.

Set a goal range, not just a single number

Most experienced marathon runners go into a race with a three-tier target:

• A goal — everything goes perfectly, you hit your training paces, the weather cooperates, you feel strong after 30km
• B goal — solid race, maybe some patches, you still run well and finish proud
• C goal — things go wrong (they sometimes do), but you finish strong and don’t blow up

For example, if your moderate predicted time is 4:15: A goal = 4:10, B goal = 4:20, C goal = finish feeling controlled and strong, even if it’s 4:35.

This matters because fixating on a single number turns every kilometre that drifts off pace into a small crisis. It also protects you from going out too fast in the first half, which is the single most reliable way to have a miserable race after kilometre 30.

Adjust your goal at key training milestones

Your goal on day one of training isn’t necessarily your goal on race day. Build in review points:

• 8 weeks out: How are your long runs going? Are you recovering within 48 hours? Do your easy runs feel genuinely easy at your planned paces?
• 4 weeks out: Have you completed at least two long runs of 28km or more? How did they feel? This is your most reliable data point.
• 2 weeks out (taper): You should feel confident, not anxious about hitting your paces. If your training runs have consistently felt a minute per kilometre harder than planned, your goal probably needs adjusting.

It’s far better to reset your B goal during taper than to blow up at kilometre 34 chasing a time that was never on the cards.

According to guidance from the NHS on safe exercise progression, building endurance gradually and listening to your body throughout training is key to avoiding injury — and an injured runner can’t race at any pace.

The Honest Takeaway

• Use a recent race result — from the last 6–12 weeks — as your baseline. Nostalgia for your old 5K PB isn’t useful data.
• Lean conservative, especially for your first marathon. The table above is a starting point. Your training volume and consistency should pull the number up or push it down.
• Your half marathon time is your most reliable single predictor. Double it, add 10–20 minutes, and work from there.
• Set a goal range (A/B/C), not a single number. It takes the pressure off your pacing and keeps the race enjoyable beyond 30km.
• Review your goal during training, not just before it. If the evidence changes, let your target change too. That’s not giving up — that’s racing smart.