Every November, I watch runners do the same thing. They take a fall half marathon time, plug it into a race calculator, and build their whole spring around the marathon number it spits out.
That number is usually too fast. Often by 10 minutes or more. And when the goal is wrong, every pace in the plan is wrong with it: the tempo runs, the marathon-pace long runs, and the first three miles of race day.
The timing matters. Boston is Monday, April 19, 2027. That is 23 weeks from today. A typical 18-week build starts December 14. The next five weeks are when you decide what your spring is actually for.
The myth: your half marathon tells you your marathon
Most race calculators trace back to Pete Riegel, a research engineer who published his formula in Runner's World in 1977 and expanded on it in American Scientist in 1981.
The idea is simple. Every time the distance doubles, you slow down by a small, fixed amount. Run a 1:30 half and the formula says about 3:08 for the marathon.
From the 5K up through the half, that works reasonably well. The marathon is where it breaks, and it breaks in the same direction almost every time.
What the research shows
Andrew Vickers and Emily Vertosick at Memorial Sloan Kettering tested this directly. Their 2016 study in BMC Sports Science, Medicine and Rehabilitation used race data from 2,303 recreational runners.
The Riegel formula held up well through the half marathon. For the marathon, it gave times at least 10 minutes too fast for half of the runners. Nearly a quarter got a prediction more than 20 minutes too fast.
They tried a gentler slowdown factor. It helped a little, but marathon pace was still overestimated for about three out of four runners.
What made the prediction better was adding training. Typical weekly mileage was strongly associated with race speed. In their model, the gap between running 50 and 30 miles a week was worth about 25 minutes in the marathon for men and about 32 minutes for women.
That is an association, not proof that adding miles buys you exactly that much. But it points at the real issue. The half marathon does not know how much you train. The marathon does.
Why the marathon breaks the formula
In 1991, Michael Joyner published a model in the Journal of Applied Physiology built on three things: VO2max, lactate threshold, and running economy. That is your aerobic ceiling, how close to that ceiling you can hold, and how much oxygen each mile costs you.
A half marathon tests those three things pretty well. A marathon adds two more that a shorter race barely touches.
The first is fuel. At marathon effort, you burn a lot of carbohydrate. Your muscle and liver only store so much of it as glycogen. When those stores run low late in the race, your muscles lean more on fat, which produces energy more slowly, and the same pace starts to cost more effort. A half is usually over before that happens.
The second is durability. Every stride loads your muscles while they lengthen to absorb impact. Over 26.2 miles, that adds up to real muscle fatigue. Your stride gets less efficient late in the race, so the same pace costs more oxygen in mile 22 than it did in mile 6.
Both of those are built mostly by volume and long runs. That is why mileage shows up so clearly in the Vickers data. A fast half tells you the size of your engine. It does not tell you how big the tank is or how well your legs hold together for three hours.
Set a goal you can defend
None of this means you should set a soft goal. It means the goal has to come from your training, not just your best race.
Here is how I would approach it. This part is coaching experience more than research.
- Start from a recent race, then adjust for mileage. If you are running under about 40 miles a week, I would not plan around the calculator number. Add real time to it. If you are holding 55 or more with long runs over 18 miles, you can aim closer to it.
- Pick a range, not a number. In November, a goal like 3:10 to 3:18 is more honest than 3:10.
- Let a tune-up race narrow it. A half marathon four to six weeks before your marathon, run on fitness you built this cycle, is a much better guide than one from last fall.
- Be honest about your marathon history. If your marathons have consistently come in slower than your half predicted, that gap is information, not bad luck.
If the goal is a Boston qualifier, this matters even more. A goal that is five minutes too fast does not just miss. It usually ends in a blowup that costs you far more than five minutes.
Work backward from race day
Once you know the race, build the calendar from the finish line back. Here is what that looks like for Boston on April 19, 2027.
- April 19: race day.
- About March 29: start of a three-week taper.
- About March 7 to 21: tune-up half marathon. This is where you lock in goal pace.
- December 14: week one of an 18-week build.
- Now to December 14: finish recovering from fall, rebuild easy mileage, keep some strides, and start strength work.
Racing earlier or later in the spring? Shift the whole thing. The order does not change.
Then put real life on the same calendar. Holiday travel, work trips, family weeks. With four kids, I would much rather know in November that a long run lands on Christmas weekend than find out the Friday before.
Ramp up without trusting the 10 percent rule
The base you build between now and mid-December is mostly easy volume. That is where the muscle adaptations that support marathon pace come from: more mitochondria, more capillaries, and better fat burning. They build over weeks and months, not days.
The usual advice is to add no more than 10 percent a week. The evidence for that exact number is weak.
In 2008, Ida Buist and colleagues ran a randomized trial with 532 novice runners in The American Journal of Sports Medicine. One group followed a 13-week program built on the 10 percent rule. The other followed a standard 8-week program. Injury rates were 20.8 percent and 20.3 percent. No difference.
In 2014, Rasmus Nielsen's group followed 874 novice runners in the Journal of Orthopaedic and Sports Physical Therapy. Overall injury rates did not differ by how fast runners progressed. But runners who increased weekly distance by more than 30 percent had a higher rate of overuse injuries like runner's knee and IT band syndrome. The authors called it exploratory, and the result fell just short of statistical significance.
My read: there is no magic percentage, but big jumps look like a bad idea. Your heart and lungs adapt faster than your tendons and bones, so the miles can feel fine before your body is ready for them. Build volume before intensity, and take a lighter week every three or four weeks.
Where the evidence is thin
The Vickers study was an online survey. The runners chose to take part, and they were younger and faster than a typical big-city marathon field. Only 156 marathoners were in the group used to test the predictions.
Both injury studies looked at novice runners, not marathoners running 50 miles a week. And nobody has run a trial on how to set a marathon goal. The best we have is good data on what predicts race times, and a lot of coaching experience on what goes wrong.
Where NXT RUN comes in
This is what PACE Plan is for. Give your coach your race date, and PACE Plan builds the season backward from it, base through taper.
When the calendar moves, and it will, tell your coach. A holiday trip, a sick week, a race you added. PACE Plan rebuilds your schedule from today forward with race day fixed, so you are not stacking missed miles onto next week. As workouts come in, your paces adjust to how they actually went, so your goal is built on your training, not a formula.
This article is general education, not medical advice. If you are coming back from an injury or have a health condition, talk with a doctor or physical therapist before building your mileage.