We become unfit much quicker than it took to get in shape. To understand how the body becomes unfit, we need to understand how we become fit.

The key to becoming fitter is to do more than our body is used to. The stress from this makes us become more tolerant, leading to higher fitness levels. Some studies show that just six sessions of interval training can increase maximal oxygen uptake (VO2 max) and improve the body's ability to fuel itself.

Cardiorespiratory fitness - shown by a person's VO2 max will decrease around 10% in the first four weeks after a person stops exercising. The rate of decline continues but eventually evens out.

Highly trained athletes (like a marathon runner) will maintain a VO2 higher than average. But the average person will fall back to pre-training levels in less than eight weeks.