I've recently become a running addict and train with a group of people who have an incredibly high fitness level. However, for two weeks within the last month, I wasn't able to exercise and now I'm afraid that I've lost all of the fitness I've built up. I am planning to start back exercising today, so I'm not worried about never becoming fit again, but it has made me curious about how quickly someone can lose their fitness. I've heard it takes only seven days, but I don't know how true this is. Can you help me figure this out?
— Running Out of Time
It looks like you under estimated the amount of time it takes to lose cardio fitness less, but just by a bit. According to Dr. Karp "within one to two weeks of stopping exercise, changes can already be seen in cardiovascular fitness." There are decreases in mitochondrial density (the mitochondria is considered the body's aerobic engine on a cellular level) and VO2max, aka maximum oxygen level consumption. Karp also added that "decreases in strength don't happen as quickly as decreases in cardiovascular endurance." I guess muscles do have more memory than the heart and lungs.
"Unfortunately," continued Karp, "It takes longer to gain fitness than it does to lose fitness. How much longer depends on a number of characteristics (age, level of fitness before taking a break, genetic responsiveness to training, etc.)." If you cannot work out at the frequency you're used to, all is not lost. If you need to decrease your workouts from five days a week to three, Karp suggested making one or two of those three days high intensity workouts.
Good luck getting back into your training and keep us posted on how the marathon goes.