acp, Picking a pace you would like to run is a mistake !!!! in my opinion, especially on your first marathon.
What would be better is look at a Heart Rate that you could sustain for x hours.
75% 80% 85% ? work out the different pace for each HR by trail and error and after a few months you will know or have an idea at least at what pace you can run your marathon, Or do the Hadd test, that will give you a good indicator
I am not a qualified coach and only have empirical knowledge, so feel free to ignore this advice.