You slept through the alarm, got stuck at work, or woke up with a scratchy throat. Sunday’s 18-miler didn’t happen. Here’s the answer: skip it, or run it at its planned distance. Don’t make it longer.
A skipped long run doesn’t hurt you. A bigger “catch-up” long run can. That one fact settles most of the panic.
Should you make up a missed long run? What the injury data says
Think of training as a savings account, not a debt. A missed session isn’t an overdraft you have to repay with interest. Your fitness is the balance, and one missed deposit barely moves it.
The risk comes from the repayment plan. In 2025, Danish researchers tracked 5,205 runners across 588,071 sessions (Frandsen et al.). About 35% got hurt. The researchers compared each run to the longest run of the previous 30 days. Runs that went past that mark by more than 10% carried a higher injury risk.
A ratio of 1.64 means about 64% higher injury risk than baseline. The middle bar sits a bit lower than the first, but the two ranges overlap, so read them as the same warning, not as proof that a bigger spike is safer. In short: a run that’s even a bit longer than anything you’ve done lately is where trouble starts.
Weekly mileage jumps and acute-to-chronic load ratios told the same study almost nothing. The single big session did.
Skipping is safe. Overreaching isn’t.
The one check: planned run versus your longest in 30 days
Here’s the math, in plain terms. Take the longest run you did in the last 30 days and multiply it by 1.10. That’s your ceiling.
| Longest run, last 30 days | Max safe long run (+10%) | A 20-mile catch-up would be |
|---|---|---|
| 12 miles | 13.2 miles | +67% |
| 14 miles | 15.4 miles | +43% |
| 16 miles | 17.6 miles | +25% |
| 18 miles | 19.8 miles | +11% |
Look at the 16-mile row. A “catch-up” 20-miler is 25% longer than your longest recent run. That lands in the spike zone.
Here’s the catch. After a missed long run, the longest run in your last 30 days is the one before the miss. So if you do the planned distance, you’re usually fine. If you try to repay the miss, you’re not.
This is a calculation from one study, not a result the study reported. It’s also not a promise. Even the evidence for weekly caps is thin. A 2014 study of 874 novice runners found that jumping weekly distance by more than 30% had a hazard ratio of 1.59, but its confidence range of 0.96 to 2.66 crossed 1, so it wasn’t statistically solid (Nielsen et al.). A half-marathon study found that a 20% to 60% weekly jump raised injuries at 21 days, but the effect faded by day 56 (Damsted et al.).
Translation: the danger shows up fast, and it comes from sudden jumps. Not from a missed day.
The decision rule by weeks to race
This is AthleteOS’s rule of thumb, built from the evidence above and coaching guidance. It’s not a published protocol.
| Weeks to race | Missed 1 long run | Missed 2 in a row |
|---|---|---|
| More than 8 | Skip it. Resume the plan. | Repeat your last completed long run, then progress. |
| 4 to 8 | Skip it. | Repeat the last completed long run once. |
| 3 or fewer | Drop it. Taper starts. | Drop both. Trust the training you have. |
| Race week or two | Never make it up. | Never make it up. |
Two rules sit under the table. First, if the miss was about schedule, you can shift the run by a day or two. Keep it away from your hard sessions and keep it at the planned distance. Second, never add a second long run to the week.
Why drop it close to the race? A meta-analysis of tapering found the best results came from a 2-week taper with volume cut 41% to 60% (Bosquet et al.). Your last big long run is typically 2 to 3 weeks out. A late catch-up run adds tired legs and no fitness.
For more on that window, see how to taper week by week and how long to taper for a half marathon.
Does one missed long run cost you fitness?
Almost nothing. Detraining research tracks athletes who stop for weeks, not athletes who skip one run. A review of the minimum dose to maintain fitness found endurance can hold for up to 15 weeks on as few as 2 sessions a week, as long as intensity stays (Spiering et al.). The reviewed studies cut training for at least 4 weeks, so one skipped run sits far inside that safety margin.
In short: your engine doesn’t rust in a week. Coach Jeff Gaudette of RunnersConnect says that for one to three missed runs, you shouldn’t make them up. He also says finishing about 80% of your plan is enough for a strong race for most runners. That’s roughly 4 of every 5 planned sessions.
Your next normal week, with its quality session, protects fitness better than cramming miles.
Why the reason you missed it matters
Schedule misses are simple. Travel, a busy week, a sleepless night. Resume as written next week. If travel is the issue, see marathon training while travelling for work.
Illness is different. Rest first. Gaudette’s advice for a long run lost to illness or fatigue is to skip it and pick up the regular long-run schedule the following week. Don’t slide the run next to a hard session just to save it.
Coming back, keep the long run easy. Coaching guidance (not trial-tested) is about 70% of planned volume after one full week off, and about 50% after two. So a runner with a 40-mile week would restart near 28 miles after a week off and near 20 after two. Our guides on missing a week of marathon training and missing two weeks of half-marathon training cover the ramp. If you’re unsure when to start again, read when to resume after getting sick.
A mini case study: Dana’s missed 17-miler
Dana is 41, training for her first marathon in 11 weeks, running 38 miles a week. Her longest run in the past month was 16 miles. Her plan called for 17 this Sunday.
A work trip ate the weekend. By Wednesday she was ready to “repay” it with a 20-miler on Saturday. That run would have been 25% over her longest. Squarely in the spike zone.
Instead she ran the 17 on the following Sunday. That’s 6% over her longest, well inside the safe band. She ran it at an easy pace, as covered in what pace your long runs should be. Over the next three weeks she stepped up to 18, then 19, each inside the 10% band, with no knee pain, which had flared in her last build. She’d skipped one rung of the ladder and still reached the top.
Missed the rung. Kept climbing.
What if you miss two long runs in a row
Repeat your last completed long run distance once. Then progress by no more than about 10% over the longest run of the past 30 days. It feels slow. It’s also how you avoid the early-weeks injuries described in why most running injuries happen in the first 8 weeks.
Run the numbers. Say your last long run was 16 miles, and the plan called for 17 and then 18. You missed both. Don’t jump to 18 or 20 to “catch up.” Run 16 once. Then step to 17.6 at most. You’ve lost one week of progress, not the whole build.
The timing of risk helps here. The half-marathon study found extra injuries in the first 21 days after a big weekly jump. That’s exactly the window a rushed return lives in. Patience in those three weeks is cheap insurance.
If you keep missing sessions, the plan may be too hard for your life. Check the warning signs your marathon plan is too hard.
How AthleteOS handles a missed long run
AthleteOS adapts your training plan when you miss sessions, travel, sleep badly or get sick, and it explains each change in plain language. You don’t need to do the math on your longest run or count weeks to race. Try it free for 14 days and let the plan do the thinking.