Long run pacing?

Hello all! I am running my first marathon next month. I do not have a coach or official training plan but I structure my weeks based on what I see on social media/here. I have been running 30-40 miles per week and my long runs are around the 18-20 mile mark.

I was wondering how I should be pacing my long runs? I usually start off easy and run the second half at or slightly faster than race pace. My goal is to average roughly 8:50 per mile on race day. If there is a general rule of thumb or strategies that have worked for anyone I’d love to hear them. Thanks!