You finished your half and now you want 42.2 km. The honest answer on timing: no study has ever measured the ideal gap. What exists is coach consensus, and it says rest 1 to 2 weeks, then build for 12 to 18.
That’s a rule of thumb, not a result. The research does tell us what to build toward, though: enough weekly mileage and one long run near 30 km.
How Long Should You Wait Between a Half Marathon and a Marathon?
Here’s what the evidence does and doesn’t say. I found no trial that compared a short gap with a long one. The 12 to 18 week range comes from coaching sites and plan writers. It’s experience, not data.
The usual coach advice looks like this:
| Your starting point | Build length | Peak long run |
|---|---|---|
| Steady running, 13 to 16 km long runs | About 12 weeks | 29 to 32 km |
| Coming back from a break | 14 to 16 weeks | 29 to 32 km |
| Low base, under 30 km a week | 18 weeks or more | 29 to 32 km |
Treat the build-length column as coach opinion. Nobody has tested it.
In short: pick the longer build if you’re unsure. Extra weeks cost you patience. Missing weeks cost you tendons.
Recovery After the Half: The First 14 Days
Racing 21.1 km damages muscle fibers. Blood markers of that damage usually peak within 24 to 72 hours. They’re back to baseline in roughly 5 to 7 days, and up to 10 to 14 days for less-trained runners. Connective tissue, like tendons and fascia, may need 10 to 14 days to remodel.
These numbers come from a secondary summary (Marathon Handbook) that cites several studies. I couldn’t check the original authors, so read them as a guide.
Translation: your legs feel better long before your tendons are ready. Walk or rest for 2 to 7 days. Then run easy.
Think of your body as a house after a storm. The roof looks fine in three days. The beams need two weeks.
Easy running after the race is fine. Hard running in week two isn’t.
What Weekly Mileage Do You Need First?
Weekly volume is one of the clearest predictors of marathon finish time in the research. A Dutch study of 441 marathon and 556 half-marathon runners used 40 to 65 km a week as the reference group (Fokkema et al., 2020). Runners under 40 km a week finished about 6.3 minutes slower. Runners over 65 km finished about 14.1 minutes faster.
The longest run mattered too. In the chart below, the reference is a longest run of 30 to 35 km.
The sweet spot is 30 to 35 km. Past 35 km you gain no speed.
Here’s the catch. Injury rates in that same study were 62.0% under 40 km a week, 53.1% at 40 to 65 km, and 41.9% above 65 km. The link between training and injury wasn’t statistically significant, so don’t read it as “more miles protect you.”
Volume isn’t the villain. Sudden volume is.
Does the 10% Rule Work?
You’ve heard it: never raise weekly mileage more than 10%. Two big studies say the rule is shakier than its fame.
In the Dutch trial, 532 novices followed either a graded 13-week plan built on the 10% rule or a standard 8-week plan (Buist et al., 2008). The graded plan didn’t reduce injuries. 20.8% got hurt on it versus 20.3% on the standard plan.
A Danish study followed 873 novices for a year. Jumps of 10% to 30% weren’t clearly worse than small ones. Jumps over 30% across two weeks carried a hazard ratio of 1.59 for distance-related injuries like shin splints and knee pain (Nielsen et al., 2014). That result was borderline (P = .07) and exploratory.
In short: the danger isn’t a 12% week. It’s a 40% panic week after you missed training. For the longer take, read why the 10% weekly mileage rule is wrong.
Spikes hurt. Small steps are fine.
The Half-Marathon-to-Marathon Build: Long-Run Progression
Coaches usually peak the long run 3 to 4 weeks before race day, then taper. They also add a down week every 3 to 4 weeks. Here’s what that shape looks like.
Run frequency matters too. In the 720 first-time New York City Marathon runners studied by Toresdahl and colleagues, 8.9% had a major injury and 48.5% a minor one. The paper itself tested a strength program. Press coverage of a 2019 sports-medicine conference (AMSSM) reported that running four or more days a week, and skipping an 18-mile run, tracked with more injury. I only saw that coverage, not the full data.
So injury was the norm, not the exception. Three or four run days with a protected long run is a sensible default.
Why Riegel Overpredicts Your First Marathon
Riegel’s formula says time two equals time one times the distance ratio to the power 1.06. Plug in a 2:00 half and you get about 4:10 for the marathon. That’s arithmetic from the formula, not a study result.
It works well up to the half. For the marathon it’s a stretch. In 2,303 recreational runners, it gave times at least 10 minutes too fast for half of them (Vickers and Vertosick, 2016). Adding weekly mileage cut the error. Running 50 instead of 30 miles a week meant 25:32 faster for men and 31:41 faster for women.
Translation: your half time predicts your marathon only if your mileage is up to it. A 20-mile-a-week runner shouldn’t bank on 4:10.
Mileage is the missing variable.
A Real-World Example
Take a runner I’ll call Dan, 41. He ran a 2:05 half on about 32 km a week, with a 16 km long run. Riegel said about 4:21. He took one easy week after the race, then picked a 16-week build.
He raised weekly volume from 32 to 56 km, never jumping more than 15% between weeks. His long run reached 32 km in week 12. He missed one long run to a work trip and didn’t make it up. He just resumed the ramp.
His marathon result: 4:34. That’s 13 minutes slower than the Riegel number, and he wasn’t hurt. This is a made-up but plausible case. Its point is the gap between the formula and a 56 km week.
When to Delay
Wait longer if any of these apply:
- A niggle still hurts on easy runs after 14 days.
- Your weekly volume is under 25 km.
- You can’t run a 16 km long run comfortably.
- Sleep or illness has wrecked the last two weeks.
Beginners face real risk. Injury rates run from 2.5 per 1,000 hours in long-distance track athletes to 33.0 in novice runners (Videbaek et al., 2015).
A late start costs a few weeks. An early injury costs months. If a missed week derails you, see how to adjust after missing a week of marathon training.
Let the Plan Do the Math
You don’t need to hand-build this ramp. AthleteOS builds your training plan backward from race day and your current base. If you miss a long run, the AI coach re-sequences the ramp and adds a down week. It won’t let you jump volume to catch up. That jump is the over-30% spike one study tied, with borderline evidence, to more distance-related injuries.
It also sets your goal from your training volume, not a bare Riegel calculation. And it gives a plain reason for every change.
Build the base first. Zone 2 running builds the aerobic base, and the drift ratio shows when your long runs are ready for more.
Wait for your legs. Then build.