Try to find a good running club nearby. I've found it a lot easier to do long runs and speedwork and even tempo runs when I've got someone to run with. Misery loves company, no?
Much of your training also will depend on what your goals are. Are you running a marathon just to have fun and finish? Are you trying to qualify for Boston or going for a PR?
If you just want to have a good time and finish under four without feeling too hammered, one of the big races will probably make it easier--more company, always someone to chase down.
If you have a specific time goal, consider choosing a race that's traditionally fast--something like St. George (Utah, in Oct.) or Tucson (in Dec.). Those two in particular have a net elevation loss and generally cool, dry weather.
For an enjoyable time, Honolulu (Dec.) is great simply because you're in Hawaii in the winter and it's warm! The Suzuki Rock & Roll Marathon (San Diego, June) is also pretty fun because of all the bands. Of course, if you're more into country music, the Country Music Marathon in Nashville (Apr. I think) would be better, but I haven't run that one so I can't say for sure. I'm more of a rocker anyways.
BTW, nowadays four-hour marathons aren't all that slow; seems like most big races especially have lots of folks coming in well after four hours.
Good luck! [:)]