There are certain training benchmarks you must reach before you can achieve a specific marathon time goal. For example, roughly three weeks before your race, you should be able to comfortably run a half-marathon training run at your goal marathon pace, with only the first vague hints of fatigue emerging in the last couple of miles.
My next marathon is 11 weeks from Sunday, and I will run my first 20-miler of the present training cycle on Sunday. I don't expect it to be a cakewalk (especially considering I'm running a 10K race the day before!), but by suffering through it I will lay the foundation for being able to coast through a 20+ miler in November.
Note that this benchmark is only relevant to runners pursuing an aggressive goal time in a marathon. If your goal is just to finish a marathon, it's enough to simply complete a 20-miler in training.