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Anyone who trains regularly knows the dilemma: What happens if I get sick, injured, go on vacation, or just need a break? How quickly do I lose fitness—especially during the critical phase of marathon preparation?

A research team from University College Dublin, led by Ciara Feely, looked into this exact question. The researchers analyzed extensive training data uploaded to Strava by nearly 300,000 marathon runners between 2014 and 2017. Within this dataset, the team identified 43,933 runners who took a break of at least seven days during the final 12 weeks before a marathon—and later completed another marathon without such a break.

The aim: to determine how training interruptions of at least one week affect race performance.

How many are affected—and how much?

The analysis reveals that training breaks are widespread—and often unavoidable.

• More than half of the participants took at least one seven-day break during the 12-week preparation period.
• About a third missed at least ten days.
• 10–15% started the race despite a two-week training gap.

The average participant was around 40 years old, trained three times a week, and covered roughly 40 km per week. Men made up the majority of the sample (233,000) with an average marathon time of 3:59 hours; women (59,000) averaged 4:24 hours. These results therefore apply to recreational runners—it remains unclear whether they also apply to elite athletes.

How much performance is lost during a break?

The analysis found measurable, but not dramatic, effects on finish time:

• 7 to 13 days off: an average of 4.25% slower
• Two-week break: about 6% slower
• Three-week break: around 7.5% decline in performance

Who is most affected by a break?

Interestingly, men experienced a greater decline in performance than women. Researchers believe that men are more likely to stick to ambitious goals, overestimate their fitness, and suffer the consequences during the race. Younger runners and sub-4:00 marathoners were also more affected by training gaps.

Not surprisingly: breaks during the peak training phase—three to seven weeks before the race—have a greater impact than those occurring earlier (eight to twelve weeks out). The tapering phase in the final three weeks was deliberately excluded.

What does this mean for your training?

While the study doesn’t provide personalized advice, it offers practical insights to help adjust your race expectations if you experience a setback:

• Short breaks (up to a week) usually have a minimal impact.
• If you miss two weeks, factor it into your goal time.
• After three weeks off, expect a noticeable drop in fitness—unless you had a solid base going in.

Conclusion

This large-scale data analysis shows that training interruptions are normal—and no reason to panic. Those who manage setbacks wisely, adjust expectations, and return consistently can still have a successful race. Even pros take breaks—so with the right mindset, your race can still be a win.