1) Thou shalt stop thinking. Believing you can run a marathon is a little like believing in fairies... the second you question your ability, everything goes up in fairy dust. You can't possibly run 26.2 miles if you think about how ridiculous it is to run 26.2 miles. It's dumb, and stupid, and weird and the rumor is the first man who ever did it died on the spot. So for now, I ask you to do yourself this little favor. Take your internal voice of reason--that thinking part of your brain that holds all logic--and just lock it away in a cage for the next few months. You won't be needing that anymore.
2) Thou shalt trust the process. If your Runner's World training plan tells you you can run a marathon, you can run a marathon. Even if you have topped out at 20 miles. Even if you took 10 walking breaks every time you ran. Even if you run really slowly. Even if you sat on the bench of every sports team you ever played on.
3) Thou shalt stop if something really, really hurts. Now let's be real for a second. Things are going to hurt when you are running for hours on end. I'm not talking about achy legs or toenails that turn black and fall off. (You need bigger shoes--a full size up from your street shoes.) I'm not talking about blisters that break and bleed through your shoes. (You need new shoes and probably new socks too.) I'm talking about a sharp pain that doesn't wear away after a couple of miles and that makes you change your stride in any way. Stop. Ice. Rest. Repeat. See a doctor if necessary.
4) Thou shalt not increase your mileage more than 10% a week. You will get injured. Promise. This also applies to trying to "make up" runs from earlier in the week all in one weekend. Your body needs rest to heal and get stronger. It is way more important to be healthy and rested than to always get your miles in.
5) Thou shalt not run a marathon to get skinny. Marathon running isn't about being a weakling and a waif, it is about being strong. Strong people eat and respect their bodies enough that they don't try to run themselves into an early grave.
6) Thou shalt take walking breaks. Some people believe that if you walk so much as a couple of minutes, you didn't actually "run" a marathon. This is the stupidest thing ever. For most people, taking a couple of short walking breaks if you need them will allow you to have a much faster finish time than slogging along even if you need a little break. It is hard to drink water or Gatorade while you're running, and if you need to stretch something out, the couple of seconds you lose while walking will pay for themselves in spades over the distance you have left to go.
7) Thou shalt be really, really proud of yourself. After you run your first mile, or your fifth, or your 25th. Every milestone counts. Every small achievement proves you are strong and brave. Just because the guy you work with runs 20 miles every weekend or because your training buddy is blabbing on about her latest fartlek, that doesn't mean that what you did isn't a big deal. Own it. Even Olympians had to start at the beginning. They had their first 5 mile run and 10 mile run, they had their milestones too.
8) Thou shalt play mind games. Don't set out to run 20 miles. Set out to run an hour. Just concentrate on that. Then when you run an hour, take a break and walk for a couple of minutes. Drink some Gatorade. Eat some Sports Beans. Then think about your next goal. It sounds silly, but it works.
9) Thou shalt be willing to make some sacrifices. More than anything else (legs of a Kenyan, endurance of a mall walker) training for a marathon takes a lot of desire and a lot of time. That means getting up on Sunday morning and running, even if your friends all invited you to brunch with unlimited mimosas. Even if you'd rather stay in bed and watch TV. Often the hardest thing is starting each run... not finishing it.
10) Thou shalt have an open mind. Even when you're tired and sore and can't walk down steps. (Yes, it's down that hurts way more than up.) You are quite literally transforming your body, and your mind will naturally want to follow. Sometimes occupying your body for hours on end is the only way to really let your mind go into those cobweb-covered corners of your brain where all of the good ideas, difficult memories, and personal truths like to hide. If you don't let yourself be open when they pop into view, you might be missing out on marathon training's biggest gift of all.