With no previous marathon experience in the bank, the best way to set the target marathon time is to run a couple of half marathon races.
When you know what you are good for on the sub-distance it is possible to make an accurate marathon time prediction. There are many good race time prediction calculators available on the web. The race time calculators predict the marathon time based on the performance in sub-distances. My own favorite can be found on the Running for Fitness web site (www.runningforfitness.org), and it calculates the predicted time based on the average of four different forecasting methods.
For the first marathon it makes sense to add some 5-10 minutes to the predicted marathon time, simply because one is not used to running the full distance. This simple rule of thumb has worked well for me. I still add about three minutes to the predicted marathon time best on half marathon performance.