Marathons are the ultimate test of both physical and mental endurance. During the marathon you may find yourself wanting to stop due to the demands it requires, but by using these tips you won't find yourself in that situation.
Before entering a marathon make sure you are in good enough physical shape to run it until the end. You must train your body to get used to running long distances which requires a built up level of endurance. Try running around your neighborhood a couple of times to get a feel for what you are getting yourself into and if at the end of your running you are still feeling good; continue on with your training. The fact is that without enough training there is no way a person can run the distance in a marathon.
After training yourself physically you must make sure the night before and even the week before the marathon to mentally go over what you have to do and think of reasons to why you want to run it and what it will feel like to cross that final line. Give yourself one die hard reason as to why you want to do this and use that reason as fuel to get yourself through the toughest part of the race.
Once you are in both the physical and mental state to run the distance try running a mock marathon a couple weeks before the competition to see if you have what it takes. If you are unable to complete the distance you set out for yourself then you need to seriously think about whether you want to do the marathon.
An optional choice if you are unsure if you can make it through the marathon is to see your doctor for a check up and have them run some tests to see if you are going to be able to go the distance. Most doctor's offices have endurance tests that they can run you through to determine your current level of endurance and asses if you are at the level required to complete the marathon.
Above all, have a positive attitude through the running, and when in doubt just remember that one reason you signed up for the marathon in the first place. Marathons are a challenge but as long as you follow these tips you will have no problem making it to the end.