Achieve a base fitness level. Do you know how fit you are right now? If you can't run, walk, bike, or do some other aerobic activity for at least 30 minutes at a time, you will need to work up to this first before attempting any running program. The first step is to talk to your doctor about a fitness assessment and to explain your intentions to begin training for a marathon. He or she will be in the best position to advise you on the challenges specific for your body and health issues. Even if you're not very fit right now, take heart. With gradual yet consistent training, your fitness will be restored quickly and you'll be able to keep improving.