Every September the same message lands in our support inbox. Some version of: I got sick, I missed a week, is my marathon ruined? Aubrie and I have coached enough runners through this exact moment to know the honest answer, which is no. But the longer answer matters, because what runners do in the week after a missed week does far more damage than the missed week ever could.
The belief that causes the trouble: fitness has a short shelf life, every day off is a day lost, so the fix is to get the lost days back fast. Both halves are wrong, and the physiology explains why.
What actually happens when you stop
The best data on this is old and still the standard. In the 1980s, Edward Coyle's lab at the University of Texas took well-trained endurance athletes and had them stop completely, then measured what changed and when. Complete rest is a harsher test than most missed weeks, where you are still walking around and living, so treat these numbers as a ceiling on what you lost.
Days one through three: nothing you would notice. Two or three days off in the middle of a hard build often makes the next workout better, not worse. A taper relies on the same mechanism.
Days three through fourteen: the plumbing changes. This is the part that scares people, and it is the part that matters least. Coyle, Hemmert, and Coggan found that after two to four weeks of inactivity, blood volume dropped about 9 percent, driven by a 12 percent fall in plasma volume, and stroke volume, the amount of blood the heart moves per beat, fell about 12 percent. Less blood per beat means the heart beats faster to move the same oxygen. That is exactly why your first run back feels awful: heart rate runs noticeably higher at the same pace, and every mile feels like you lost a month.
You did not. Plasma volume is one of the fastest-responding systems in the body. It drops in days and it rebuilds in days once you start running again. The engine is fine. The fuel line is temporarily narrower.
Weeks two through eight: the engine itself. In Coyle's time-course study, VO2max fell about 7 percent over the first 21 days of complete rest and settled 16 percent below the trained level by day 56. The aerobic enzymes inside the muscle that let you burn fuel with oxygen dropped with a half-life of about 12 days. That sounds bad until you read the next line: after 84 days off, those enzymes were still 50 percent above the level of people who had never trained, and capillary density in the muscle had not dropped at all.
That is the whole point. Those runners had stopped for twelve weeks and were still fitter than sedentary people in the ways that take years to build. One week does not touch that. Your capillary network, your mitochondrial base, your tendons and bones, your running economy, none of it is going anywhere in seven days.
The honest caveat: endurance fades faster than fitness
There is one place where the reassurance runs out, and a marathoner needs to know about it. VO2max is not the same thing as the ability to hold a pace for three hours.
A 1993 study by Madsen and colleagues had trained endurance athletes cut back to a single session a week for four weeks. VO2max held steady. But their time to exhaustion at 75 percent of VO2max, which is roughly the effort of a marathon, dropped 21 percent. The engine was the same size. The ability to run it for a long time had eroded.
Iñigo Mujika and Sabino Padilla's two-part review of detraining in Sports Medicine draws the line at about four weeks. Under that, most of what you lose is quickly recoverable. Past it, you are into real structural loss. So the practical rule is this: one week off, nothing meaningful is gone. Two weeks, you have lost a little endurance and it comes back with two or three long runs. Three or more weeks, especially if the weeks you missed were the ones with your longest runs, is when it is fair to revisit your goal time. Not the race. The time.
Where the real damage happens
If the missed week costs so little, why does it end so many fall marathons? Because of what comes next. Two mistakes, and I have watched runners make both in the same week.
Mistake one: making up the miles. The plan said 55 last week and you ran zero, so you run 60 this week to catch up. Your body does not care what the plan said. It cares what you have actually been doing, and what you have actually been doing for the last seven days is nothing. A large prospective study of novice runners, led by Rasmus Nielsen and published in the Journal of Orthopaedic and Sports Physical Therapy in 2014, found that those who increased weekly distance by more than 30 percent over two weeks had a clearly higher rate of distance-related injuries: runner's knee, IT band syndrome, shin splints, patellar tendon pain. Those are the injuries that take you out for six weeks, not one.
Tim Gabbett's work on training load in the British Journal of Sports Medicine puts a number on the same idea. He compares your acute load, roughly the last week, against your chronic load, roughly the last month. Injury risk stays low when that ratio sits between about 0.8 and 1.3, and it climbs when the recent week spikes well above what you are accustomed to. A week at zero followed by your biggest week of the year is exactly that spike. The application to runners is still being refined and the ratio is a guide rather than a law, but the direction is not in dispute. Sudden jumps in load are where injuries live.
Mistake two: testing yourself on day one. You feel fine, you are anxious, so you go straight into the tempo run to see where you are. Heart rate is high, the pace feels terrible, and you conclude you have lost a month of fitness. You have not. You have low plasma volume. But now you are demoralized, and you either dig deeper to prove something or spiral into a mid-September panic.
How to come back
The plan below assumes a plain missed week from illness, travel, or life. If you missed the week because of an injury, this is a different conversation and the pain, not the calendar, sets the pace of your return.
If you were sick
The neck rule still holds up. Symptoms above the neck only, like a runny nose or a mild sore throat, are generally fine for easy running if you feel up to it. Fever, body aches, chest congestion, or a deep cough means you wait. Come back once you have been fever-free for 24 hours, and come back easy. Running hard through a fever or a chest infection is the fastest route from a one-week layoff to a six-week one, and in rare cases it can involve the heart. Not worth it, at any goal time.
The one-week return
- Days one and two: easy runs at about 60 to 70 percent of your normal easy-day distance. Run by effort, not by pace. Ignore your watch. Your heart rate will be high and your pace will be slow, and both are temporary.
- Days three and four: normal easy runs, and add strides to one of them. Four to six relaxed 20-second pickups wake the legs up without adding any real load.
- Day five onward: rejoin the plan where it is on the calendar, not where you left it. Do not shift the whole schedule back a week. Your race date did not move.
- First hard workout back: run it, but cut the volume by about a third. Four by mile instead of six. The paces will come back faster than you expect once the blood volume returns.
- First long run back: cap it at 75 to 80 percent of what the plan called for. If the plan says 20, run 15 or 16. The next one can be full.
Total volume for that first week back should land around 70 to 80 percent of normal. That keeps you out of the spike zone and back on schedule within days.
The two-week return
Spend one full week at about 60 to 70 percent of normal volume with one moderate session and a long run at around two-thirds of the scheduled distance. Then rejoin the plan. Two weeks off, five weeks out, is still a race you can run near your goal. It is not a race you can run on the original plan without editing it.
If you can only partially train
Sometimes the missed week is really a compromised week: three runs instead of six because of travel or work. Robert Hickson's studies in the early 1980s found that people who cut training duration by as much as two-thirds held their VO2max for 15 weeks, as long as they kept the intensity. Cutting intensity while keeping volume did not work nearly as well. So when you have to trim, trim easy mileage and protect the one quality session.
The math for a BQ runner
Take a runner five weeks from Chicago, chasing a 3:00 to qualify for Boston. A missed week costs, at most, a few percent of aerobic capacity, most of it back within days. At the start line that is somewhere between nothing and a minute, and a good taper hides it. A stress reaction from cramming 60 miles onto zero costs the race. The patient runner who trims the first week back is going to Boston. The one who tries to prove the week never happened is going to a physical therapist.
Where NXT RUN comes in
Most of what goes wrong after a missed week is a scheduling problem. The plan on your calendar was written before you got sick, and it does not know. That is the job PACE Plan was built for. Tell your coach you were out for the week and back on Monday, and it rebuilds the schedule from today forward with your race date fixed. The missed miles do not get piled onto next week. The long-run progression picks up at a sensible point. The first workout back is sized for a runner who has been off, not one who was peaking two weeks ago.
Adaptive Pacing handles the other half. Your first runs back come in slower than the plan expected, and instead of holding you to targets set before you got sick, your upcoming paces adjust to what your body is doing now. As the blood volume returns, the targets follow.
None of that replaces judgment. But it does mean the week after a missed week is a conversation with your coach instead of a guess made at 6am.
This article is general training information, not medical advice. If you missed training because of an illness that involved chest symptoms, a high fever, or anything that has not fully resolved, check with a healthcare professional before returning to hard running.