how to pace long runs for sub 4 marathon goal

hi! i am running NYC marathon this year. this is my first marathon and my initial goal was just to finish… now i’m hoping to actually finish at a sub-4 or low 4 hr time. i run my long runs at anywhere from a 10 min - 10:45 min mile pace, shorter runs are usually some where in the 9min mile range. when do i need to start picking up the pace on my long runs to prepare for marathon day where i will hopefully maintain a 9 min mile pace? thank you!