How hard is the training for a marathon for someone who has never done it before?

Depends on your goal: Buddy with an experienced person who plans to go at a pace that would be suitable for you. They can guide you on warm ups, preparation, & schedule of gradual increases in distance toward the goal. Start with smaller/shorter distance events. Be reasonable & listen to your body so you don't overdo it or get hurt. Make sure your health is good (consider checking with your pcp).

Challenging, doable: Many have done it ! you must start off slowly. You develop a pretraining routine to prepare you for your training. Once you are comfortable with this, you begin to train. There are many excellent sources for plans, and many groups to train with - esp in nyc. See ny road runners club. Browse the bookstore and check the training books / diaries. Good luck !