Being fit is one of the most important aspects of Martial Arts. If you are not fit you will not be able to get through training without great discomfort and mental effort. In the long run this will drain you and may cause you to quite.
Build an adequate level of fitness within your Martial Arts program by starting with a very small dose of exercise. The smaller the dose the better. Then start increase the dose gradually, the slower the better. They key is to remain consistent and never miss a day.
Please make sure that whatever type of exercise you choose you first make sure you know to study the proper form needed. Form is above all most important, many people make a big deal over exercise selection, exercise selection is important but second to form. Bad form will lead to injury, so please make sure to seek a professional that can insure you have good form.