How to Choose a Realistic Marathon Goal from Your Half-Marathon Time

A half-marathon result can help you choose a marathon goal, but recreational runners should treat the predicted time as a candidate to check against their training. Use a recent, representative race result, then assess your endurance preparation, practiced fueling, and the conditions of your target marathon before committing to its pace. If those pieces do not support the prediction, choose a slower target or a finish-focused plan.

The tricky part comes when the calculator produces a time you badly want to run. A strong half marathon may make a marathon milestone look close, while your long runs tell a less convincing story. Here is how to weigh both without turning the final weeks of training into an audition.

What does your half-marathon time actually tell you?

It tells you what you could sustain over that race distance on that day. Translating it into a marathon time requires an assumption about how much you will slow as the distance doubles.

That assumption matters. In a 2016 study of 2,303 recreational runners, Andrew Vickers and Emily Vertosick found that a commonly used race prediction formula substantially underestimated marathon times. Its predictions were at least 10 minutes too fast for half of the runners assessed. Models that incorporated training mileage improved prediction.

That finding does not mean you should add exactly 10 minutes to every calculator result. It means the relationship between shorter-race speed and marathon performance varies enough that a formula alone is a weak basis for your starting pace.

Give your half-marathon result more weight when it reflects your current training and was a sustained race effort. Give it less weight when it is an old personal best, came before a substantial training interruption, or was run under conditions very different from your target marathon.

Write down the context alongside the finish time: the race date, course, weather, effort, and whether you maintained your pace late. A number with that context is more useful than a personal best copied from your watch history.

Calculate a starting estimate, then question it

One common method is the Riegel formula:

Predicted time = known race time × (target distance ÷ known distance) raised to a fatigue exponent.

Use the same distance units for both races and the same time units throughout. The Vickers and Vertosick study evaluated an exponent of 1.07, with additional analyses using 1.06 and 1.08. For the examples below, the assumed exponent is 1.07.

A marathon is 42.195 kilometers, according to World Athletics, and a half marathon is half that distance. The distance ratio is therefore 2.

For a hypothetical 1:45:00 half marathon:

105 minutes × 2^1.07 = approximately 220.44 minutes, or 3:40:26.

These are calculated examples using that formula, rounded to the nearest second. They assume the same fatigue exponent for every runner; they do not account for individual training, course conditions, fueling, or stops.

The formula and its limitations come from the recreational-runner prediction study. Different calculators may use different exponents or models, so their outputs can disagree even when you enter the same result.

There is also a separate kind of calculator: one that converts a chosen pace into a finish time. RaceThere's marathon finish-time calculator performs that arithmetic and displays splits. It answers what a pace produces over the distance. You still need to decide whether that pace is realistic for you.

Does your training support the predicted pace?

Look for agreement across your training record. A convincing goal should have more behind it than one fast race or one excellent workout.

Review the weeks you actually completed

Open your log and review the marathon-specific part of your build. Note your usual weekly distance, running frequency, long runs, and interruptions. Use completed training rather than the mileage printed in the plan.

The prediction study found that typical weekly mileage helped estimate marathon performance. Treat that as evidence to consider, not a reason to make a sudden mileage jump. Changing next week's training cannot retroactively supply the preparation missing from the previous months.

Training plans also differ in their expected starting fitness. The Boston Athletic Association's marathon plans offer four levels selected using running frequency, weekly mileage, and long-run distances. Even its novice plan begins around 25 miles per week. Those plans are specifically designed for Boston, but they illustrate why the label “beginner” tells you less than the actual training requirements.

Ask whether your chosen plan matched your starting point and whether you completed its main progression. Several missed weeks deserve more attention than one missed easy run.

Examine your long runs beyond the average pace

For this decision, record how the run unfolded. Did you maintain a manageable effort? Did you finish the planned distance? Could you follow your fueling plan? What happened in the final portion?

An easy long run does not need to match marathon goal pace to be useful. Equally, running a long run unusually fast does not automatically validate a faster goal. Look at the run's intended purpose and whether you completed it with control.

If your plan already included marathon-pace sections, use them as supporting evidence. Repeated trouble holding the candidate pace is a reason to reconsider it. Completing those sections comfortably supports the goal, while still leaving uncertainty about the full distance.

Include the days afterward in your assessment. A workout that required you to abandon the next several planned runs should not carry the same weight as one you absorbed within your normal routine.

Check whether your race plan has been practiced

Your target assumes you can keep moving while eating and drinking as planned. Review what you actually managed during long runs: the products, timing, carrying method, and any stops.

If you repeatedly skipped fuel, struggled to take it, or needed lengthy stops, resolve those details before choosing the fastest plausible prediction. For a detailed fueling schedule, use your training plan or qualified support; the goal here is to identify whether your proposed race execution has evidence behind it.

How should you handle conflicting evidence?

When your half marathon looks strong but your endurance preparation is uneven, give the training gap meaningful weight. A calculator can make your shorter-race performance look precise without resolving that gap.

Use this reference table as an editorial decision aid, rather than a validated scoring system:

Do not count a hot race, a missed week, and a difficult long run as interchangeable penalties with fixed minute values. They create different uncertainties. Describe the issue first, then decide whether you have enough evidence to retain the candidate goal.

A worked decision: is 3:40 realistic after a 1:45 half?

Consider a hypothetical runner whose recent half marathon was 1:45:00 on a course broadly comparable to the target marathon. The formula estimate is 3:40:26, so the runner begins by assessing a goal near 3:40.

In the first scenario, the runner completed a consistent build, managed the long runs, practiced fueling, and handled the marathon-pace work already included in the plan. Keeping 3:40 as a candidate is reasonable. The final decision still depends on race conditions and how the runner feels approaching the event.

In the second scenario, the same half-marathon result is followed by several disrupted weeks. Long runs repeatedly end early, and attempted goal-pace sections become strained. Here, 3:40 rests mostly on the half-marathon result. A slower target is better supported.

Suppose this second runner evaluates 4:00 instead. Using pace = time ÷ distance, 240 minutes ÷ 42.195 kilometers gives approximately 5:41 per kilometer, rounded to the nearest second. This calculation assumes that average includes all stopped time. It explains what four hours requires; it does not establish that four hours is achievable.

The runner should assess that slower pace using the same evidence. If it also lacks support, a finish-focused plan remains the more useful choice. The point of revising the target is to find a pace the preparation supports, rather than simply replacing one attractive round number with another.

Should you run another race to confirm the goal?

Only if it fits the training plan and serves a clear purpose. A tune-up can provide a current result or let you rehearse race execution, but adding a hard race solely to obtain reassurance creates another demand on the build.

Decide beforehand whether the event will be raced for a result or used as a controlled workout. Those purposes produce different evidence. A half marathon deliberately run at marathon effort is useful practice, but its finish time should not be entered as though it were your best half-marathon performance.

When comparing races on RaceThere, check the organizer's course information and timing within your plan. A convenient event is useful only if its purpose matches the training you need.

If you are already tapering, review your existing evidence instead of adding a final proving session. You do not need another test to acknowledge that the goal should change.

Set the final goal for the race you will actually run

Before race day, review the organizer's course information, forecast, and aid-station arrangements alongside your training. Where conditions differ materially from those you practiced, leave room to revise the target rather than applying an invented universal adjustment.

Write a short decision note: your candidate time, the evidence supporting it, and the circumstances that would make you change course. Include a finish-focused fallback that describes actions—following your practiced fueling plan, using planned walk breaks, and adjusting effort when needed.

Once you have chosen a defensible target, RaceThere's race-day pacing guide can help you think through execution. Keep goal selection and split planning in that order: first decide what your preparation supports, then plan how to run it.

Find a race on RaceThere that fits your preparation and gives you a useful next goal.