If you are new to marathons, you may find it helpful to run longer distances with one or two friends or with a larger group for safety as well as for socializing. Seek out running clubs in your area to find company for your weekly long runs. Not only do some of these groups arrange to place water in strategic locations, but they will know where you can safely run 15-20 miles.
If you prefer to run by yourself, use the local club as a resource to help you plot a route for a long run if you aren't sure where to go. Also, many local running clubs can recommend local sports medicine professionals, such as podiatrists and chiropractors, who are sympathetic to runners.