Depending on the level of fitness you want or are required to be at for your level of football, it is best to find out what level of fitness you are at right now. If you are just starting out, you can do this by either following the tests in the Guide to Fitness or by attending a fitness session or local gym.
The FA Guide to Fitness was given to all referees on a Basic Course as part of their 'Welcome to Refereeing' Packs. Alternatively, it can be downloaded via the link in 'Related Documents'. In this section there is also a guide to a 13-week pre-season training plan and a nutrition guide. As you will see, the first part of the fitness schedule is two weeks of rest. The training sessions are based on reaching a specific heart rate range, therefore they can be applied to any level of fitness as long as the heart rate percentages and rest periods are adhered to.