Isn't it amazing how fitness can take months and months to develop, and yet be lost in little more than a week!? Fitness is gained incrementally i.e. we get a little bit fitter every time we push ourselves beyond our comfort zone, it’s a process known as supercompensation. During supercompensation your body will gradually increase the size of muscle fibres, escalate the production of red blood cells, and improve the efficiency with which you store and mobilise glycogen (stored glucose) for energy.
The body is reactive to stimuli, so if an external stimulus (such as running) consistently places strain on your cardiorespiratory system, your body will physiologically shift in order to cope with this. Take that stimulus away for a while, such as during injury, and the body begins to downregulate accordingly.
It’s great to see hard work paying off, but unfortunately our bodies can revert back to base level in next to no time. There are two main types of fitness, aerobic fitness (cardio) and structural fitness (bones, ligaments and tendons), and although both decline during prolonged periods of inactivity, they do so at different rates. Cardio fitness will begin to deteriorate in less than 14 days via a process known as detraining, and structural fitness may take a hit over the same time period. Mercifully it seems the fitter you are, the longer this detraining process will take!
There are some nutritional provisions you can put in place to slow this process and optimise recovery including increasing protein, maximising vitamins and minerals via fruit and veg or a multi-vitamin and mineral, and ensuring adequate anti-inflammatory foods such as omega-3 from the likes of oily fish or flaxseed are consumed.