It turns out that even with the great effort we put into training, taking a bit of time off can mean that we become "unfit" much faster than it took us to actually get in shape.
To understand how the body becomes "unfit," we first need to understand how we become fit. The key to becoming fitter -- whether that's improving cardiovascular fitness or muscular strength -- is to exceed "habitual load." This means doing more than our body is used to. The stress that this has on our body makes us adapt and become more tolerant, leading to higher fitness levels.
The time it takes to get fit depends on a number of factors, including fitness levels, age, how hard you work and even environment. But some studies do indicate that even just six sessions of interval training can lead to increases in maximal oxygen uptake (V02 max) -- a measure of overall fitness — and improve how efficiently our body is able to fuel itself using the sugar stored in our cells during exercise.