Marathon Pacing What Time to Expect in the Marathon

I have got a question about marathon pacing. I've been working out for a good 3 years now on and off. In high school I ran cross country and played soccer. My best 5k was 18min something.

For the past month I've been running / cross-training about 6 days a week for at least and hour a day. I always have plenty of energy left after my runs and when I jump rope for an hour or play soccer. I feel great.

I signed up for a marathon (a month from now), and I'm wondering how long it might take me to finish. Right now I can finish 5k runs in the 18 min range, and once a week I run a 10 mile trail.

– Benjamin, Berkeley, CA, U.S

Answer by Dom: Hi Benjamin, thanks for the question. You're fitter than most people who write in, but fitness and marathon-readiness aren't the same thing, and I want to show you exactly why with your own numbers.

1. A reality check
2. What your 5k time actually predicts – and why it's misleading here
3. Your plan for this marathon
4. A better way to train for your next one

A Reality Check

Apologies if this comes as a shock. You are underprepared for a marathon. Six days a week of running and cross-training, an hour a day, with energy left over – that's genuinely good general fitness. But a marathon isn't won or survived on general fitness. It's won on the long run.

Your longest run right now is a 10-mile trail once a week – about 16k, or roughly 38% of the marathon distance. Ideally you'd have built your long run up to 2.5-3 hours, around 20 miles, backed up by a mid-week run of at least 90 minutes. You have neither, and you have four weeks to find it in, which isn't enough time to build it safely.

What Your 5k Time Actually Predicts – and Why It's Misleading Here

Let's use the numbers properly, because this is where most runners get it wrong. Using a standard race-time prediction formula (Riegel's formula, which most marathon calculators including mine are built on) and plugging in your 18-minute 5k, the raw prediction comes out at around 2 hours 53 minutes for the marathon.

Ignore that number. It's not a real prediction for you, and here's exactly why.

Race-prediction formulas assume you've trained specifically for the target distance. They convert your current speed into an equivalent time at another distance assuming your endurance can support it. A 5k, at 5k, is far too short to tell you anything about your endurance over 42k – it tests speed and a few minutes of discomfort, not the ability to keep moving for three-plus hours. The longer the race you use as your predictor, the more reliable the conversion. A 5k is the weakest distance you could possibly use, and for someone with your training gap it's actively misleading – it flatters you.

Your 10-mile weekly trail run is a far better data point than your 5k, and it still only gets you to 38% of race distance. Realistically, factoring in the endurance you haven't built, I'd expect somewhere in the 4:00-4:45 range, and it could easily run longer if the last 10k turns into a battle – which, on this training, it likely will.

Your Plan for This Marathon

Given you haven't done the training for it, your best bet to finish is to run conservatively from the start, potentially with walking breaks built in from kilometre one – not held in reserve for when things go wrong. Try alternating 7 minutes running with 3 minutes walking.

Ignore time goals for this one. If you go out chasing anything close to that 2:53 number, the second half will fall apart and you'll be forced to walk it in anyway, just later and less comfortably.

A Better Way to Train for Your Next Marathon

First, spend time in the Marathon Training Tips section on this site, including the Q&A. You'll see quickly that proper marathon training is tough, and the distance deserves respect.

If you want to race a marathon properly next time, build your running until a 13-mile long run is comfortable and repeatable – you should have done several 13-15 mile long runs without issue. Only then start an 18-week marathon training block. What most of us don't realise is that you need to train to get ready for marathon training, before the marathon training itself even starts.

When that base is there, a race-prediction calculator becomes genuinely useful – and a longer race, a half marathon rather than a 5k, will give you a far more honest number. Even then, the prediction only holds if your training backs it up.

Sorry if this has been a sobering read, but I hope it helps.

Best of luck.

Kind regards,
Dom

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