we all know why you should get in shape, why you should be healthy, fit, etc. we all make our best efforts to do so with the least amount of negative impact on our lives. that is, take the least amount of time away from the other things in our lives - work, play, family, friends, etc - and the least amount of financial and to some extent bodily impact. for many, we even do our investing in fitness at one phase of life- youth, middle age, whatever - and hope it pays dividends later in life...like now. in other words, we often refer to what we used to/could have done and pretend to live off those glory years - or months, in some cases - as if somehow those hard-earned capabilities would persist beyond that phase of training.
the truth is, we de-condition much faster than we get in condition, and while we often retain some extended benefits from a training period - for ex, strength declines slowly largely because the nervous system, which is the first and most persistent adapter to a strength training routine, has "memory" (some would say, muscle memory) that enables muscle recruitment even when hypertrophy withers.
other benefits, such as aerobic or anaerobic capacity, fade, too. since the complex biochemical changes, not just the gross anatomical ones (like heart ventricle size), reverse engines to their previous state of dormancy or half-use, our capacity to perform for long periods of time (aerobic) or in short bursts of higher intensity (anaerobic) diminish, too.
the magic question for years has been, how little do i need to get in shape? and how little do i need to do to stay there? and while there is no precise answer to those questions, there is at least one caveat that persists beyond all the research studies: if you don't use it, you lose it. furthermore, if you do use it, and stop using it, it won't be there when you want it again. and this article discusses this issue a bit deeper: http://well.blogs.nytimes.com/2010/12/29/phys-ed-if-you-are-fit-you-can-take-it-easy/?ref=health
so, whatcha waitin' on; get up and start exercising...again.
oh, and happy new year, too.