Cellular damage post marathon is best measured by the presence and production of creatinine kinase (CK) - a marker that indicates damage to skeletal and myocardial tissue - and increased myoglobin levels in the blood stream.
One study concluded that CK damage persisted more than 7 days post marathon while another study discovered the presence of myoglobin in the bloodstream for 3-4 days post race. Both of these studies clearly indicate that the body needs rest post marathon to fully recover from the cellular damage caused during the race.
Unlike muscle soreness, these markers of hard training and racing aren't always noticeable. This is why you need to take downtime after a long training segment or marathon, even if you don't feel sore.
Finally, studies have shown that the immune system is severely compromised after running the marathon distance, which increases the risk of contracting colds and the flu.
A suppressed immune system is one of the major causes of overtraining. Therefore, skipping a much needed rest period could lead to interrupted training down the road, which could significantly derail your long-term goals.
What Elite Runners Do
While scientific evidence helps support training assumptions, perhaps the most obvious example of the importance of taking downtime is exhibited by elite runners. Elite runners are advised by the best coaches in the world and their livelihood depends on consistent training and racing.
Professional runners who make their living running races still take downtime after marathons and long training segments. For example, Dathan Ritzehein blogged about the need for downtime after a long training stint and Alberto Salazar confirmed that star pupils Galen Rupp and Mo Farah would be taking two weeks off from running after their successful track seasons.
Why Taking Downtime Won't Negatively Affect Your Fitness
It's not hard to persuade a runner to believe that a marathon or long training segment is difficult on the body. However, it's quite another to convince the same runner that taking seven to 10 days off to rest up won't hurt their fitness. While it may same counter-intuitive, research has proven that resting for 7 to 10 days will not significantly diminish fitness.
The Science of Rest
Because VO2 max is one of the best measurements of a runners physical fitness, it's the most useful baseline to compare the effect of detraining on your aerobic system. To be brief, VO2 max is an individual's maximum ability to transport and use oxygen during exercise.
Recent studies show that there is little reduction in VO2max (1 to 3 percent) in the first six to seven days following inactivity in well-trained runners. Furthermore, even after two weeks of not running, studies show that VO2 max decreases by only 6 percent.
While percentages are fantastic, what do those numbers really mean for runners? Let's use an example of a 20-minute 5K runner. A 20-minute 5K runner has a VO2max of roughly 49.81 ml/kg/min. After seven to 10 days of no running, the hypothetical 5k runner would lose about 3 percent of his or her VO2 max. Accordingly, after downtime, his or her new VO2max would be 48.49 and he or she would now be in 20:30 shape.
Anecdotal Evidence From Elite Runners
Luckily, this slight reduction in fitness is easy to gain back. Again, using elite runners as an example (since their training and racing schedules are widely available and we can assume they are training optimally), it only takes three to four weeks for runners to return to hard training and near peak racing shape.
Meb Keflezighi might the best example to illustrate how quickly a runner can return to peak fitness. After the 2012 NYC Marathon, Meb was forced to rest for three weeks thanks to an untimely foot infection. With just 70 days to prepare for one of the biggest races of his life - the 2012 Olympic Trials - Meb regained his fitness quickly to dispatch one of the most heralded fields in US history and punch his ticket to the London Olympics..
How Long Should You Plan to Rest
Most coaches and elite runners suggest you should take off one week after a 5K training cycle, seven to10 days off after a 10k or half marathon, and a full two weeks off after a marathon. It might sound like you would be holding yourself back by being so cautious, but your long-term progression will actually benefit.
Good luck at your upcoming race and don't forget to schedule a little rest and relaxation time afterward.race.