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.
Another important benchmark is what I call the 20-mile cakewalk. This benchmark works as follows: By the time you begin your two-week pre-marathon taper, you should be capable of "jogging" 20 miles so easily that you could play a game of pickup basketball later the same day and wake up the next morning with minimal muscle soreness. To reach this benchmark, you should aim to run your first 20-miler roughly 10 weeks before your marathon and complete at least three more training runs of 20-24 miles in the next several weeks.
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.