More on fitness
Many sports oriented systems prefer that you achieve a high level of fitness to progress. In fact, if you are infirm in some way (such as a missing limb, blindness) then it becomes harder still. Generalities aside, choosing a martial system does not necessarily mean you will get fit.
Choosing a martial art is more about turning up for two hours a week and following a hobby. You can do this (and noone will stop you) but you are missing out on more than you would realise. Training in a budo art is about following a way of life. In Ninjutsu, the emphasis is on survival (see the articles section).
Of course, by being fitter and eating more appropriate foods will aid your survival but these are things that you can pursue outside of the dojo. It is upto you to change how you eat and when and where you exercise. Hopefully, by practising a martial art you will have gained the self esteem, confidence and discipline to carry it out.
Age
Traditional systems, or those with their foundation in martial tradition, emphasise the ability for the student to train for the whole of their lifetime. The art changes subtly as they age and as their bodies alter how they can do things.
It should be obvious that a student aged 50 is not necessarily as physically resilient as one who is 17.
Obesity
Many people, certainly in the western world, are now obese. Whether from food, excessive drinking or some other life factor. Being overweight should not prevent you from starting a martial art. However, given the strain your body is already under, it would be advisable to lose some weight and seek the advise of a doctor first.
But remember training will not necessarily make you thin.
Higher self esteem, increased confidence and self control will help you find a shape, size and lifestyle that you are happy with.


