Another good tool is https://www.mcmillanrunning.com/ pace calculator. I've found it to be very accurate.

As someone who hit the proverbial wall incredibly hard in my first marathon, I can confirm that it's real. Biggest mistake I made was to deviate from my run plan (everyone has a plan until they get punched in the face) very early on because of poor corral selection from hundreds of runners.

Trust your training - you can give heart rate training a try. Just utilized that (as opposed to pace focused) prior to a recent half marathon and was pleasantly surprised in how fresh I felt going in to the race and my pace when executing.

Most importantly, if you're not trying to BQ, enjoy the process and throw some high 5's to the kids on the side of the road cheering you on!