Updated December 29, 2014.
Once you've run your first race, or you've been running for a few months, people may start to ask if and when you'll ever run a marathon. Completing a 26.2 mile race is no small feat and, as marathon runners can tell you, the training -- not the race itself -- can be the hardest part.

I think that any healthy individual who is willing to commit to the training can complete a marathon, but I don't recommend that new runners jump right into the marathon distance.

That's an easy way to get injured and possibly end up hating running, rather than developing a life-long healthy habit. If you've never run before, it's important to first feel very comfortable with running, get your body used to the physical and mental demands of running, slowly build up your mileage base, and run some shorter distance races.

So, how do you know if you're ready to start training for a marathon? Here are some questions to ask yourself:

1. How long have you been running?
It's best to be running for at least six months (one year is better) before you start training for a marathon. If you have that experience, your body will have already made some physical adaptations and you'll be more physically and mentally prepared to handle the rigors of marathon training. Sure, many people jump right into marathon training and run a marathon several months after they first started running. But many people who attempt to do it that way get injured or burned out during the training.
Also see:  7 Steps to Injury Prevention

2. How many miles have you been putting in?
A good marathon training base is 15- 20 miles per week for a few months before you start training for a marathon.

Your body needs time to adapt to the pounding that it will be taking during the training. And make sure you're not running on the treadmill for 100% of the time because your body makes different adaptations for road running. When building your base, try to run at least 1/3 (more is better) of your miles on pavement, as that's likely what you'll be running on during the marathon.
Also see:
Tips for Running Farther
How to Start a Running Habit

3. Did you talk to your doctor?
Even if your doctor cleared you to start running for exercise, it's still important to discuss your marathon training plans with him or her. Your doctor may suggest additional tests or have specific exercise recommendations before clearing you to get started with marathon training.

4. Have you ever run a race?
Before you start thinking about running a marathon, it's a good idea to try out a few smaller races, like a 5K or 10K. You'll be able to figure out if you truly like training and racing before making a huge time, emotional, and possibly financial commitment. I always like to see runners complete a half marathon before they do a marathon so they get a better sense of what's involved with the training and race itself and to decide if they truly enjoy long distance running!
Also see:
How to Find Local Races
Tips for Your First Road Race

5. Do you have time to train?
Some people don't realize that marathon training is very time-intensive. At times, it may feel like a part-time job (that you don't get paid for). Beginners should plan to run or exercise at least 4-5 days a week and one of those days will involve hours of running (at the peak of your training). Think realistically about your work, family, and other responsibilities to determine if you would have the time to commit to the training. Some people may need to get family members on board before committing if they're going to need help with childcare and/or household responsibilities.
Also see:
How Parents Can Find Time to Run
How to Make Running a Priority
How Long Does It Take to Train for a Marathon?