Well, there's definitely a lot of debate about this issue. But most running experts will tell recreational marathoners that it's not a good idea to run more than 20 miles at one time during training. The reason is that running 20+ miles really takes a toll on your body. You'll need a long recovery period and you run a high risk of getting injured. Running 18 to 20 miles as your longest training run will prepare you to complete the marathon. So, the potential negative effects of running longer than 20 miles really out-weigh any possible benefits, such as feeling more mentally ready to run 26.2 miles.
During the two weeks before your marathon, you'll be cutting back your mileage. This tapering period will allow your body to recover from all those months of training. You'll feel rested and ready to take on the 26.2 mile distance.
Also, it's important to remember that being prepared for your marathon is not about just one long run -- it's about the consistent training you've been doing for months. If you've been following your marathon training schedule, you'll be ready.