Every running plan comes back to three numbers: how far, how long, and how fast. Fix any two and the third is decided. That simple relationship is the foundation of pacing, and once you understand it you can plan workouts, pace races, and predict finish times with surprising accuracy — all from a calculator and one honest recent effort.
This guide walks through how pace works, why an even effort beats a fast start, and how a single formula turns a 5K result into a credible marathon target. None of it requires a coach or a lab. It requires only that you respect what the numbers are actually telling you.
Pace, speed, and the arithmetic that links them
Pace is time per unit of distance — minutes per kilometre or minutes per mile. Speed is the inverse: distance per unit of time, like kilometres per hour. Runners think in pace because it maps directly onto a watch face during a race. If your goal pace is 5:00 per kilometre, you glance down at each marker and know instantly whether you're on track.
The conversion between the two units trips people up more than it should. A mile is 1.609 kilometres, so your per-mile pace is always a larger number than your per-kilometre pace — you're covering more ground before the clock ticks over. A 5:00/km pace is roughly 8:03/mile. Rather than doing that arithmetic mid-race, let a tool hold both figures at once.
The third piece is splits: the projected time to reach each checkpoint if you hold a constant pace. Splits are the practical output of pace maths. They tell you what the clock should read as you pass 5K, 10K, and halfway, so you can catch a too-fast start before it wrecks the back half.
Why even pacing wins
The most common racing mistake is going out too hard. It feels effortless in the first few minutes because adrenaline and fresh legs mask the cost, but every second banked early is usually repaid with interest later. Physiologically, starting too fast pushes you above your sustainable effort, accumulates fatigue and lactate sooner, and forces a painful slowdown in the final third.
The evidence from world records and well-executed marathons points the same way: the fastest times come from even pacing or a slight negative split, where the second half is marginally quicker than the first. That approach keeps you just under your limit while your body is fresh and lets you spend the reserve when it counts.
This is why the splits table matters. If your goal marathon pace is 5:00/km, the table shows a 25:00 5K and a 1:45:35 half. If you cross those markers meaningfully faster, you're borrowing from your finish. Discipline in the first 10K is what makes the last 10K possible.
The Riegel formula: predicting a race from a race
Predicting how a time at one distance translates to another is where things get genuinely useful. In 1981, Pete Riegel published a model that has held up remarkably well:
Predicted time = known time × (new distance ÷ known distance) raised to the power 1.06.
The exponent is the whole story. If pace never changed with distance, that exponent would be 1.0 — double the distance, double the time. But pace does fade as distance grows, because you can't hold your 5K effort for a marathon. The 1.06 exponent encodes that fade: it makes longer races proportionally slower in a way that matches how real runners perform.
Work a quick example. A 20:00 5K predicts a 10K of about 41:42 — not 40:00, because doubling the distance costs slightly more than double the time. That extra 1:42 is the exponent doing its job.
Where prediction is reliable — and where it isn't
Riegel's model is most trustworthy when the target distance is within about two to three times the race you actually ran. Predicting a 10K from a 5K, or a half marathon from a 10K, lands close for most trained runners. The physiology is similar enough across those ranges that the curve holds.
Stretch too far and the prediction quietly becomes a fantasy. A blistering 5K predicts a marathon time that assumes you have the endurance base to sustain that relative effort for 42 kilometres — the long runs, the fuelling practice, the durability. Without that specific training, you will run considerably slower than the raw number suggests. Treat a marathon predicted from a 5K as a ceiling on a perfect day with perfect preparation, not a promise.
Conditions matter too. The formula assumes similar terrain, weather, and freshness. A hilly trail race won't convert cleanly to a flat road time, and a hot day inflates every prediction. Use a recent, hard, evenly paced effort on comparable ground as your input, and the output earns your trust.
Turning predictions into training paces
A prediction is a starting point, not a training plan. Once you have a realistic goal time, divide it by the distance to get goal pace, then build sessions around that number:
- Easy runs should sit well slower than goal pace — this is where aerobic fitness is built, and it should feel conversational.
- Tempo runs live near your threshold, comfortably hard, teaching your body to clear lactate at a faster clip.
- Interval work runs at or below goal pace in short, repeated bursts to raise your ceiling.
The mistake here mirrors the racing mistake: running easy days too fast. If every run drifts toward goal pace, you arrive at race day tired rather than sharp. Most of your weekly volume should be genuinely easy, with a smaller dose of quality work.
Pace only tells half the story of effort, though. On a hot day, up a hill, or when you're tired, the same pace costs more. That's why pacing by heart rate — or at least sanity-checking pace against it — keeps easy days honest.
A simple race-week routine
Put the pieces together and race week becomes calm and mechanical:
- Set the goal. Take a recent race, run it through the predictor, and pick a target time that respects your training — not your ego.
- Build the splits. Convert the goal to pace and read the split for each checkpoint on the course. Write the key ones on your hand or wrist.
- Bank nothing early. Hit the first checkpoints exactly on pace or a hair slower. If it feels too easy, good — that's what the first third should feel like.
- Spend the reserve late. If you reach three-quarters distance on pace and still have something left, that's when a negative split turns a good race into a great one.
The honest limits of the numbers
Calculators and formulas describe patterns; they don't know your training history, your sleep, or the wind on the day. A prediction that assumes even pacing can't account for a poorly judged start, and a splits table can't run the race for you. The value is in the framing: they give you a realistic target and a plan to execute it, which removes guesswork and the temptation to wing it.
Used that way — recent effort in, sensible goal out, disciplined execution on the day — pace maths and race prediction are among the highest-leverage, lowest-effort tools a runner has. They cost nothing, they take seconds, and they consistently turn a hopeful guess into a race you can actually run.