Half Marathon Training: What Time to Expect?

I am a beginner attempting to run my first half marathon. On average, I can run a 10k at a 9-minute-mile pace. I run every other day with distances from 3-7 miles, with cross-training on the alternates. I want to know what an average time would be for me at age 49 and for my son who is following the same schedule at age 13. Thanks in advance. – Johnathan, Evans, GA

Hi Johnathan,

Great to hear you and your son are tackling this together — running alongside your kids is one of the best things you can do for both of you.

Here's what we'll cover:

1. How to predict your half marathon time from your 10K pace
2. Why the right training — specifically the long run — determines whether that prediction holds
3. What your son at 13 can realistically expect

1. Predicting Your Half Marathon Time from a 10K

A widely used rule of thumb for converting a 10K time to a half marathon prediction goes like this: double your 10K time, then add 10–20 minutes. Faster runners add around 10 minutes; slower recreational runners add closer to 20.

Let's work through your numbers. A 10K at 9 minutes per mile puts you at the finish in roughly 56 minutes (9 min × 6.2 miles). Double that is 112 minutes. Add 20 minutes and you land at 132 minutes — just over two hours ten, or around 2:12.

You can verify this with the Race Conversion Calculator, which uses a slightly more sophisticated formula but will arrive at a similar figure. A two-hour-ten to two-hour-twenty finish looks like a reasonable target — in principle.

I say "in principle" because there's a big condition attached to that prediction. The formula assumes your training has prepared your body for the full distance. Specifically, it assumes you have done the long runs. And that's where I want to focus, because it's where most first-timers come unstuck.

2. Why the Long Run Is the Most Important Variable

You mention running 3–7 miles every other day. That's a solid base — you're moving regularly, and the cross-training on alternate days is sensible. But here's the thing: for a half marathon, it's not your average weekly mileage that matters most. It's how far your longest run goes.

The half marathon is 13.1 miles. If your longest run in training tops out at 7 miles, you are essentially asking your body to go nearly twice as far as it has ever been — on race day, when you're also dealing with race-day adrenaline, crowds, and the temptation to go out too fast. That's a recipe for a very uncomfortable last 5–6 miles.

I speak from experience here. My first half marathon was run on a maximum long run of 10 miles. I thought that was close enough. The last 5K taught me it was not — I was running on fumes and it hurt. A lot.

The fix is straightforward: get at least 2–3 long runs of 10 miles or more into your training before race day. If time allows, push one of those up to 11–12 miles. See the increasing mileage safely guide for how to build up progressively without picking up a niggle — the general rule is no more than 10% increase in weekly mileage at a time.

If your race is close and there simply isn't time to get those long runs in, then adjust your goal. Run more conservatively — aim for 2:30 rather than 2:12 — and you'll have a much better experience. A conservative first half marathon almost always beats an ambitious one that falls apart after mile 9.

A good training plan will structure all of this for you. The half marathon running program is built for beginners who have completed a 10K and want a structured build-up to their first half — well worth a look given where you are right now.

3. What About Your Son at 13?

This is a genuinely interesting question — and the honest answer is that the formula works for him too, but with a few caveats.

Younger runners typically have higher natural VO2 max values than older runners. At 13, your son's aerobic engine is already impressive, even if he doesn't know it yet. If he's been running the same distances as you on alternate days, his 10K time is likely a bit faster than yours — and the same double-plus-20 formula gives a reasonable half marathon estimate.

The practical difference is that he may well beat the prediction rather than fall short of it. Young runners often have a naturally efficient stride and recover more quickly between efforts. The caveat is the same as for you — the long run matters enormously. Running 13 miles for the first time on race day, whatever your age, is a shock to the system.

Encourage him to treat the last 5 kilometres as the race. Everything up to mile 8 is just getting there. If both of you can arrive at mile 8 feeling comfortable and in control, the finish line will take care of itself.

Running a half marathon together at 13 and 49 is something neither of you will forget. Focus on the experience first — the times will look after themselves.

Kind regards,

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