I just ran my first marathon about 5 weeks ago.
I am not a fast runner, but I stick to it. My advice to a new runner is to go at your own pace, and if you feel like you cannot run another step then slow down a bit and see how much longer you can go. Worry about total time spent running first, then worry about speed once you can go a few miles without stopping.
Some people like to do run/walk intervals forever, not just when they are first starting. It helps them net faster times and run longer distances. I prefer to run a bit slower and not take walk breaks, so it's just personal preference.
Oh, and never forget that a huge part of distance running is mental. Your physical training will only take you so far if you doubt yourself mentally. You have to believe you can run x number of miles before you will be able to, no matter how well conditioned you are physically.
Just be careful - running is addictive!!
