Before undertaking a multi-day hike it pays to be in good shape. Why? Two principal reasons:
Enjoyment: The fitter you are the less you struggle, physically and mentally. This particularly holds true during the first few days of a hike. By being in good shape, you are able to focus more on the beauty of your surroundings and less on the distraction of how exhausted you feel.
Health: A good level of pre-hike conditioning minimizes the likelihood of stress/repetition related injuries such as knee and achilles ailments.
For a detailed look at hiking fitness, see HEALTH & SAFETY .