Some things I learned from my first marathon and some others I've learned since (many have been mentioned already but I wanted to share my experiences):
First and foremost accept that you will make mistakes both training for and running your first marathon. Unless you are a pro moving up in distance it is improbably you won't make mistakes. You just want to limit the mistakes to things that won't cause injury or DNF.
The biggest mistake I made was concentrating on the long run at the expense of the cumulative miles. I mistakenly thought if I get my longest run in each week the other runs could be skipped. Nope.
As already mentioned but I can not stress this enough, put in plenty of slow miles. I trained way too fast not only for my first marathon but in general my first decade of running. I started doing slow miles in 2018 and while it took a little while to notice the impact I can tell you without reservation that it has improved my speed, endurance, and reduced my injuries.
Use training to try out fueling strategy, what you feel comfortable in, any areas that become problematic (chaffing, blisters, hot spots) so you can work on a prevention strategy.
Those were really the big lessons learned from my first marathon. Things I've since learned as I tried to decrease my marathon times:
Miles are king. Since reading the
Hansons Marathon Method I'm a believer in cumulative miles and training for the fatigue you feel later in the marathon. My current plan starts week 1 with 30 miles and until December it prescribes 6 days a week ever week.
Schedule your runs like you schedule anything else in your life. If you have a family calendar put it on the family calendar so you have the time dedicated to it. If you are doing it over lunch at work mark yourself away. Don't let chance play a part on whether or not you get in your run. That being said...
You can be flexible, we all have other obligations, but be smart about it. Don't treat any single day as an island, especially if you are using a training plan that includes runs tailored to different needs (speed, tempo, repeats, easy). As an example, lets say you have a plan that has Wednesdays off, hard track days on Thursdays, recovery runs on Fridays, tempo runs on Saturdays, and then long runs on Sunday keep those in mind as a series. If you push the hard track day from Thursday to Friday keep it followed by a recovery run, don't skip right to the tempo run.
Do bodywork and work on mobility. I like to foam roll, do a little Graston using
this tool, and use a lot of the mobility exercises in Kelly Starrett's
book and Mark Cucuzzella's
book.
Lastly, if you have a goal time in mind the best advice I've heard is to use a good race equivalency calculator and make sure you can hit the shorter time goals that correlation to your marathon goals. I like
this one. If you want to run a 3:45 marathon make sure you can run a 23:37 5K, a 49:01 10K, and a 1:48:40 half. If you can't work on that first and then work on the marathon training.