I’ve done both – running the AJC Peachtree Road Race with almost no preparation and running it after spending weeks getting ready. Not surprisingly, running a long race, make that any race, goes better when you prepare adequately (especially the Peachtree, where it will be hot and humid and the weather will shock your system if you aren’t ready for it).
Ten or 11 weeks prior seems like as good as time as any to begin training. For me, that means printing a schedule of training runs from runnersworld.com rather than just running however far I feel like going. (There is a free option and several pay options from Runners’ World and many other websites have pay plans. Nike also has free plans available through Nike+. Note that most of these plans seem to recommend 12 weeks to fully get ready.)